The TLDR; is treating the GPL as an issue of contract law rather than copyright, and because of contract law, a third party can sue instead of just the author.
Vizio makes smart TVs that use GPL code and they haven’t shared the source.
In October of 2021 the Software Freedom Conservancy (SFC) decided to launch what is believed to be the first significant open source lawsuit based in contract rather than in copyright. Critically, the SFC’s case argued that anyone who benefits from the General Public License (GPL), not just the authors of the software, should be able to bring a lawsuit to enforce the terms of the GPL.
Open source and AI related copyright, licensing, and contract arguments all in one year!