The Federal Arbitration Act explicitly permits. There are a variety of situations where arbitration entirely makes sense: companies may not want to deal with court resolution of contract disputes for efficiency reasons (the courts are slow and expensive) or may want to have a defined venue with rules of their choice (I've been told that, for international deals, foreign firms are _very_ wary of US discovery rules, and agreeing to arbitration instead can let you avoid them and smooth signing the deal, which the parties probably care more about than potential disputes in the future). It's a very useful and reasonable option for contracts _between peers_

Unfortunately, the law treats negotiations between actual peers and large companies "negotiating" with individual customers the same way--it'd be hard to delineate distinct classes of actors that can and cannot enter into arbitration agreements (or contracts in general).

Requiring an explicit opt-in versus "by continuing to use this service with saying otherwise, you consent to this change" for certain classes of contractual changes is probably doable (IANAL this could possibly raise some issue I'm not thinking of), but in practice there's no political will to make that change. Automatic consent is legal currently and will likely remain legal in the future.