I'm not sure whether this clause would stand up in court, especially if AT&T were discovered to be doing something skeevy that only a class-action lawsuit would

I'm not sure whether this clause would stand up in court, especially if AT&T were discovered to be doing something skeevy that only a class-action lawsuit would provide effective relief for. However, in general, a lot of consumer contracts have mandatory arbitration clauses buried in the fine print and the courts have been happy to delegate their work to the arbitrators. 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.