A bit of legal theory here.
When a case is filed, a party brings a complaint, which sets forth allegations constituting claims for relief. The allegations are factual in nature but are generally stated.
A party who has been sued ultimately responds to the complaint by filing an answer in which he admits or denies the allegations made against him. In the normal case, most material allegations are denied.
The allegations that have been denied form the factual issues in the case, which ultimately must be decided by a trier of fact at a trial. When the facts have been so determined at a trial, the law is applied to them to determine whether one or more claims has been proved as to all of their elements (liability and damages or other relief sought) and, if so, judgment is entered on such claims against the wrongdoing party.
The time between the filing of a complaint and the date of a trial is typically a year or more.
Between the original filing and the trial, discovery is conducted by the parties. Discovery is done through a range of legal mechanisms that allow each party to discover what evidence underlies another party's case. This can be through written interrogatories, document requests, admission requests, depositions, etc. The idea behind discovery is to find out what admissible evidence exists to support the allegations that are at issue in the case.
After a reasonable period has passed for each party to conduct discovery, any party can move for summary judgment if it wishes. This is a tough motion to win. In essence, a party asking for summary judgment is telling the judge that, based on all the undisputed evidence that will be before the court in connection with the motion (evidence submitted by the moving party and by the opposing party), the moving party will be entitled to judgment "as a matter of law." If the court agrees that there are no material undisputed facts relating to all the essential pieces of a claim in the case, or if the undisputed facts show that there is an insuperable barrier to relief on a party's ability to prove a claim, the court can make a legal determination that such a claim be disposed of then and there and without need of allowing that claim to reach trial.
At a trial, it is normally up the "trier of fact" to resolve all factual disputes framed by the justiciable issues in the case. That usually means a jury (assuming one or more parties has demanded a jury trial). And, since the right to a jury is constitutionally guaranteed, a judge has no authority to supplant the authority of a jury to determine factual disputes at the trial. If no one has demanded a jury, or if the nature of a claim is such that is not triable to a jury, then the judge acts as the trier of fact in lieu of a jury. In either case, when a case presents disputed material facts that need to be resolved before it can be decided, the resolution of such facts is always left for trial and cannot be short-circuited by a judge before trial.
Thus, when (as in this case) a judge is presented with a summary judgment motion, the judge cannot dispose of any claims without first finding that the material facts relating to the claim being ruled upon are undisputed. If, in the course of the motion, a party presents evidence that, if believed at trial, would support that person's position and potentially support a judgment in that person's favor, then the judge must hold that disputed facts exist that preclude the claim's being dismissed summarily before trial.
That is why summary judgment is so hard to get. In virtually every case that is fought in court, a party will have something to present in terms of evidence that might allow that party to win at a trial. The evidence may not be strong or particularly credible, but a court has no power to weigh the strength of evidence in deciding whether to enter summary judgment. If contradictory evidence is presented, and is material to an issue, then summary judgment is automatically denied.
In this case, Viacom basically admitted that, with respect to every take down notice involved in the case, Google had acted promptly to remove the offending material. The judge also acknowledged that Viacom had presented evidence that (if believed) suggested that Google knew that there was widespread copyright infringement on its YouTube site and that it even encouraged such infringing activities so as to profit from them. Having so assessed the evidence, the judge interpreted the DMCA in a way that this latter evidence was irrelevant to any issue in the case - that is, it was not material to the resolution of the infringement claims that Google might have wanted to profit from infringing activity because all that mattered under DMCA is that Google always acted quickly to do the take downs whenever Viacom flagged an offending item. Having so construed the law, and given that Viacom had no evidence to dispute the fact that Google at all times acted promptly to do the take-downs, the judge held as a matter of law that there was nothing left for a jury to decide at a trial and that the judge could therefore enter a judgment on such issues summarily before trial for Google and against Viacom.
In this context, a judge would not have any particular incentive normally to avoid entering summary judgment just to try to make any ultimate ruling more appeal-proof. If the judge here, for example, had denied the motion and let the case go to trial, this would simply have put a jury through a useless exercise of trying to determine certain facts that the judge did not regard as relevant anyway. In the end, thinking as he did, this judge would have wound up with the same result in any case.
When considering incentives generally, though, it is true that is generally "safe" for a judge to deny summary judgment in most cases, even when a party has otherwise made a pretty compelling case, because that decision is not appealable and the net effect of the denial is to force the parties to go to trial. In such a case, in order to protect his record and avoid the potential for reversal on appeal, a judge essentially "kicks the can down the road" and fails to do the right thing that would save all parties a great deal of time, effort, and expense. This does unfortunately happen and is a source of great frustration to litigants in some cases. It also means that meritless cases can get to a full jury trial, where the pressure to settle is great just because a party would rather pay at that point rather than take unnecessary risks with what a jury might do.