This just becomes a question of pragmatism at that point - Google lacks the capacity to determine which of the blocked results are legal versus are not without incurring cost, so the most realistic approach is to recognize that a majority of results from that domain are illegal and block the domain. This is just the simplest way to enforce a particular rule in a particular case that can't otherwise be cheaply codified programatically.
Yet Google doesn't generally block most content that is illegal in one jurisdiction in all others where it is not illegal. If Google is deciding to do that just with TPB, then that is indeed an adhoc decision.