>Got a source on that?
The source is the CFAA, which makes it a crime and/or a tort to commit any "unauthorized" access to a computer system. Because authorization is not defined in the statute, it's a matter of interpretation whether or not one's use is unauthorized. Historically, judges have strongly disfavored scrapers.
Most boilerplate Terms of Use contain language that forbids all "spiders, scrapers, bots, and all other automated means of access", or something along those lines. Most companies assert that accessing any page beyond the front page of their site constitutes a binding agreement to their ToU, and thus that any automated access is "unauthorized". Scrapinghub appears to be of the opinion that browsewrap agreements are unenforceable, and while some judges have agreed with that, some haven't.
Beyond the argument that scraping is a breach of contract (violating their Terms of Use) and that since you agreed to that contract, you understood that automated access was unauthorized, there's the potential criminal element, which was deployed against weev for exposing a minor data leak in AT&T's system and against Aaron Swartz for exceeding MIT's authorized access to JSTOR and downloading publicly-funded academic data (including data which was out of copyright). You basically just have to really hope that no one inside the company you've "wronged" is good friends with a prosecutor.
Because there is a lot of grey area around what may or may not constitute "unauthorized" access to a computer system, if a company does bring a tort claim against you for accessing their system without authorization, you might actually win -- if you can afford the time and money to fight them for the minimum 3-5 years it'll take your case to resolve. This is hundreds of thousands in legal fees easy.
3taps eventually had no choice but to give up because they couldn't take the legal costs anymore, and Power Ventures tried to stick it out and ended up not only being held liable for $3 million in damages to Facebook's systems when no actual damage had occurred at all, but the veil was pierced and the founder held personally liable. It's obvious from the court documents that he was struggling to afford counsel, and companies must be represented by an attorney, so he didn't even have an option to try to represent himself.
>Google scrapes all the time. This is how they index all the pages it discovers.
Yes, Google's operations are, strictly speaking, illegal on various fronts. They depend heavily on automated access, which many sites they index explicitly forbid and thus Google is committing "unauthorized access" to these computer systems, and they also store complete copies of the site and the individual images displayed on the site, virtually all of which are protected by copyright, and all of which constitutes flagrant violation of copyright law.
If someone did bring a CFAA claim against Google for this (which no one would, because Google is one of the wealthiest companies in the world, and it'd therefore cost tens of millions to sue them), Google would likely argue that robots.txt is the only authorization it is obligated to observe, which may or may not be an effective argument. Google also make no guarantees about the extent to which it obeys robots.txt; it's a way to signal your desires to Google, which it may or may not honor.
tl;dr The very short answer to all of this is that traditionally, the legal system has been extremely suspicious of scrapers and has treated them very badly, applying concepts intended for the physical world like trespass to chattels to server access. This has been improving somewhat in recent years, but is still a very financially and legally precarious situation in which to find oneself. The people who get away with it get away with it because no one sued them before they were too big to sue.