E.g. they don't get to sue you for using it and not opening your own software that links to it, since you can buy a license allowing you to do just that.

That's also why some GPL software is dual licensed. GPL for the masses, and a proprietary license allowing you to do whatever without needing to follow the GPL if you can afford it.

Ah! You mean a special kind of proprietary license for them. That makes sense.

I thought you meant they would just use copyrighted code that wasn't under a GPL (which would be just as illegal and probably more dangerous in terms of enforcement).