They'll stop doing that if most code is GPLv3. If most code is GPLv3, we're all better off – but while people are still prepared to work around GPL-rejecting companies, those companies can still reject the GPL.
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).
Do they forbid using the code, or incorporating the code into their own works? I hear of places forbidding the use of GPL code (even in-house) where they use proprietary software like Microsoft Word with no worries.
No, they might not. Just like Microsoft, they might demand that you don’t release your plugins. Addidionaly, GNU software gives you the alternative of releasing your plugins. But you don’t have to, and they can’t make you do it.
They allow them. I had cases where clients forbade some GPL code (banks, etc) - worse with GPL3.