They do but not doing so is a breach of contract not a copyright violation.
What (at least in the US) makes it a copyright violation is the clause in the GPL contract which causes a automatic termination, and in turn makes you end up without a (previously existing but breached) contract granting you usage rights. Which then can be used to sue you based on copyright law.
But everything which causes the then not to be reached causes you not to be able to sue based on copyright but only based on breach of contract.
Through in practice this kinda could make a lot of copyright/licensing a joke (not just for software, but also e.g. art, movies etc.).
They do but not doing so is a breach of contract not a copyright violation.
What (at least in the US) makes it a copyright violation is the clause in the GPL contract which causes a automatic termination, and in turn makes you end up without a (previously existing but breached) contract granting you usage rights. Which then can be used to sue you based on copyright law.
But everything which causes the then not to be reached causes you not to be able to sue based on copyright but only based on breach of contract.
Through in practice this kinda could make a lot of copyright/licensing a joke (not just for software, but also e.g. art, movies etc.).