I am happier when I see copyleft but let’s be honest, I would contribute to an interesting, useful project regardless of their choice between MIT and GPL. Same for companies: some prefer MIT, but there is no way they are not going to contribute to the Linux Kernel just because of copyleft. So bottom line is: make something that people enjoy/find useful and see contributors flocking.

CLAs are a different matter: I do not contribute to projects which ask you to assign them copyright unless I 100% trust the organisation behind them.

File an issue in their repos, sometimes people (understandably) do not understand licencing very well — or it might be they were granted an exception.

If that fails you can contact the library author and the repositories who host the code.

Great suggestions in this discussion! Rather than adding my favourites, I will add some resources that list more games.

• Libregamewiki: it is really comprehensive (sometimes too much, including even not-so-good-games). They care about licencing and is is very easy to browse, top-notch for me.
• Open source games: a more relaxed repository, with lots of material.
• bobeff open source list: this is curated, which means that there are not so many games but each and every one is stable, good, maintained.
• Arcane Cache: a fantastic blog with reviews of libre games — or more precisely, underground games, there is a lot of discussion on how gamedevving philosophy too. The reviews are always in-depth and allow you to experience the games on another level, and each game is a small jewel in its category. Strongly recommended!