Also remember that companies are happy to put all sorts of unenforceable terms into their legal boilerplate, to intimidate customers who don't know better.
If I were writing the laws I would enact a very strong presumption against mandatory-arbitration clauses in any contract of adhesion. But I'm not.
Until it's tested in court though we'll likely never know. I assume part of AT&T's rationale is that:
A) They have the time & resources to argue then appeal this point for many years should any significant class action arise.
B) It may be enough to discourage consumers from joining aclass action. In many class actions the only people to see significant money are the lawyers. Consumers usually receive something of nominal value and if AT&T were to highlight this cause it may be enough to persuade most people not to bother.
In any case, I fear that you're very correct on both points. Even if it's not legally enforceable, it's going to cost the average consumer more than they're worth to fight it and have it nullified.
Also, there's a difference between an illegal contract (which is a nullity and for which no remedies are available in case of a breach) and an unenforceable contract (which may still be enforceable under theories of unjust enrichment, promissory estoppel, etc). A contract to do something illegal is an illegal contract. A contract with unconscionable (per se unfair) terms is usually an unenforceable contract.