What's the point of the GPL if you can't enforce it, is a more interesting question.
Although the busybox example has been heralded as a "win" for GPL, as far as I can gather there hasn't actually been a judgment (except in the case of Westinghouse which was bankrupt and did not defend itself), merely a succession of "settlements" - in other words it was cheaper to comply with the terms of the licence than to pay lawyers to pursue the case. There have been some cases in Germany, but it's very difficult to find out what actually went on.
The real problem with the GPL is that the loss suffered by the copyright owner is largely intangible: the infringer may profit from misuse but that is not money that would otherwise go to the rights owner - it would still go to the infringer provided they complied with the licence terms. Getting real damages in an actual case that made it through the judicial process would, I think, be quite difficult. And we will probably never know, because it's unlikely to be worth anyone's time or money to take a case to a legally-certain conclusion.
If the only contribution these lawyers are making is to threaten companies with legal costs fighting suits that may or may not have any merit, in the hope that people will simply settle for a quiet life, how exactly are they any better than Prenda Law?