This is a very dubious assertion. I'm not aware of any evidence that mass surveillance deters or prevents crime at all, much less is "an essential tool" for doing so.
But even if one, for the sake of argument, concedes the point that mass surveillance does significantly deter or prevent crime, you still have a system set up where the costs of that surveillance (loss of privacy, loss of accountability for abuses of power, introducing/secretly discovering backdoors, etc) are borne by the least powerful--ordinary citizens--while the benefits of mass surveillance (concentration of power, ability to bribe/extort/intimidate rivals, being seen as "doing something" about terrorism, etc) accrue only to those who are already powerful.
That is the real problem with mass surveillance. It creates a positive feedback loop that only exacerbates existing power imbalances, inevitably leading to corruption and capricious injustices by those who are most able to get away with it. Having a speedbump on the road to that inevitable destination, even a big one, is not much of a consolation if the heading is still the same.
Oh, you think you are sneaky. This is so carefully worded. You explicitly twist the words of the GP, where they use "fighting crime" you turn that to meaning "deters of prevents crime." They mention "surveillance", and you turn that into "mass surveillance."
That's like me saying "I don't see how fingerprint analysis helps to deter or prevent crimes." Oh sure, it helps capture people after the fact, but I don't think there has ever been evidence show that fingerprint analysis has actually deterred or prevented crime.
So, while you can stand their, smug with your "technically correct" remark, the reality is "surveillance is an essential tool in fighting crime" has been proven to be correct time and time again, and has been instrumental in handing convictions for a long, long time.
However, you also don't provide much proof that "surveillance is an essential tool in fighting crime". Its efficiency to deter/prevent crime, or even to help catch offenders a posteriori, is also highly debatable (I guess it's more likely to be helpful for the latter, but that still makes the definition of "fighting crime" rather vague as well).
For kicks, I like this paper on how street-lighting and CCTV cameras are apparently equally efficient as deterrent: http://onlinelibrary.wiley.com/doi/10.1111/j.1745-9133.2004.... .
In the case of digital surveillance, I'd be extremely cautious about its actual efficiency. And that something is legal and efficient doesn't necessarily warrant that its ethical and should be used.
If this is to be analogous to bulk surveillance, the assassination agency would have to discover that the target had actually died in an accident eight months ago, just before the forms were submitted, and they would simply retroactively dismiss any wrongful death or homicide cases that may be ongoing.
The panopticon operators are asking forgiveness rather than permission, and only for those specific instances where the surveillance needs to be laundered from illegally gathered to warrant-authorized.
As such, I have no faith that the spying has ever stopped, or slowed, or even decreased its rate of growth. The steep bureaucratic hurdle does nothing to prevent it. The most it does is limit the amount of information that can be moved from the shadows into the sunshine via that method, and strongly encourages less costly alternatives to actually honoring the law. One such workaround is "parallel construction", where the illegal spying is converted into an anonymous tip to another "innocent" agency of the state, who then gets reasonable suspicion on a pretext charge--like failure to signal a lane change, plus the invocation of the magical ritual phrase "I smell marijuana"--and then the road-patrol cop somehow finds 20 kilograms of cocaine in the trunk.
Fixing or eliminating the FISA court won't stop the behavior. Only actual accountability for the people actually doing the dirty deeds will help.
State and federal law enforcement can request a warrant. The turnaround time is far lower, the search methods more diverse, and the evidence gathered can be used in court.
Unfortunately, state law enforement can still subpoena your email metadata and your phone records without a warrant: https://en.wikipedia.org/wiki/Smith_v._Maryland
That sounds perverse.