Most Terms of Use contain language that states that automated access is unauthorized, and most companies allege you've agreed to their Terms of Use by accessing any page other than their front page (or accessing the front page more than once). Furthermore, even if this is lacking, once a company provides notice that you are no longer authorized to access their computers, it doesn't matter what the Terms of Use may say or whether the ToU is considered a binding contract; you've received notice that your access is unauthorized.
The CFAA generally applies to any computer access, as the Commerce Clause has been interpreted as applying to completely intra-state transactions (ignoring the fact that most computer access crosses state lines, which would make it a federal issue anyway), and thus, effectively all computers attached to the internet are "protected computers". Even if you're able to prevail in the argument that the CFAA is not applicable, there are similarly-worded state statutes, and as mentioned, other issues may federal matters, like alleged copyright and trademark infringement (which is usually alleged in such cases).
The CFAA is somewhat grey so if you have a few hundred k or more to flush on your lawyers, go ahead, it's possible that you may win. For most people, it's not relevant, because they can't afford to go head-to-head against a major global corporate entity like Amazon. One of the main goals of such a company is to make the lawsuit as costly as possible for the opposing party, since they have infinity money and they know that their opponent likely doesn't.
If you're interested, there have been a handful of cases already litigated on the CFAA. The scrapers usually get smoked hard. Look up Ticketmaster v. RMG Technologies, Craigslist v. 3Taps, Register.com v. Verio, and Facebook v. Power Ventures to get started.