Despite what they may put in the TOS, there are certain inalienable rights which take precedence over anything you could possibly sign. For instance, microsoft could not enforce a clause which states "if you use this service you agree that your sister will be raped once per month by a microsoft employee". You cant just sign your life away because you agreed to use a product.
As far as I know, class action suits are actually one of these. All microsoft can actually uphold is that you agree to TRY arbitration first. Obviously the goal here is to exhaust the user since arbitration would require you to fly to Microsoft's headquarters and set up a discussion with their lawyers within their jurisdiction. BUT, if you actually did this, and failed to reach an agreement, THEN you could sue.