In my understanding, they could not, just as they could not force any other software pirate to do what they please. It's just tht opening GPL-violating code is

In my understanding, they could not, just as they could not force any other software pirate to do what they please. It's just tht opening GPL-violating code is probably a cheaper alternative than paying damages (and as far as I know, there's a legal precedent for a GPL violator paying damages to the authors of the pirated code). Alternatively, a GPL violator could negotiate a proprietary license for the same code from the author(s) of the GPLed code they used - it's questionable whether (a) you can hunt down all the authors, since open source licenses specifically enable collaborative development of software and (b) whether all the authors would agree to such a deal, especially after the company in question tried to pirate their code without giving anything back in return.