Linux

Not really because their rights have not been violated, nothing was stolen from them. They were presented with a software product that had a limited license, and they accepted that. As far as they are concerned, the developer has fulfilled their contractual obligation to them; they were never offered a GPL license so they got exactly what they were offered.

The author of the GPL'd code however is another story. They wrote software distributed as GPL, Winamp took that code and included it without following the GPL. Thus that author can sue Winamp for a license violation.

Now if that author is the only one who wrote the software, the answer is simple- Llama Group pays them some amount of money for a commercial license of the software and a contract that this settles any past claims.

However if it's a public open source project, it may have dozens or hundreds of contributors, each of which is an original author, each of which licensed their contribution to the project under GPL terms. That means the project maintainer has no authority to negotiate or take payments on their behalf; each of them would have to agree to that commercial license (or their contributions would have to be removed from the commercial version of the software that remains in Winamp going forward). They would also each have standing to sue Llama Group for the past unlicensed use of the software.

That depends a lot on how the license gets interpreted and how license violations are handled by the local law. The argument for why the end user cannot do anything about GPL violation is that the violated contract is between upstream and the "bad" developer - the upstream project gave the bad developer access to their source code under the condition that the license stays the same. You as the end user only get exposed to the bad developer's license, so you can't do anything. It's the upstream who must force them to extend a proper license to you.

However there was also a case recently where the FSF argued that this interpretation / handling of the situation is against the spirit of GPL and I think they won, so... Yeah, it's just unclear. Which is normal for legal texts (IMHO intentionally, but I'm not here to rag on lawyers, so I'll leave it at that).