Technically that's true. But courts won't just say "contract of adhesion! *void*" to any non-negotiated contract. The "take-it-or-leave-it" aspect is just one factor to consider. I strongly doubt any court would refuse to enforce the contract of sale of a video game because it was not negotiated.
The point stands that the terms of service is not bargained for. But a better argument is that the buyer does not even see the terms until after the contract of sale, and so there was never any contract for the ToS to begin with. Plus, any unusual terms in form contracts aren't going to be enforced, since courts recognize that no one reads them (my Contracts professor even defined form contracts as "documents which are irrational to read"). This is the same reason why you should never actually bother reading through credit card forms, etc.
This is getting somewhat off-topic, though. I doubt Blizzard will be sued over this. My main point is that, if true, this is a remarkably dumb policy.