AGPL does not do anything to force people to contribute their changes back. They have to make the changes available, however that's very different from actually contributing back. It is not hard to make a low quality contribution that the upstream would for very good reason refuse even though it 'works'.

The poster child for this is chromium which is GPL because it has clear and obvious roots in the old KHTML code of 20 some years ago. In theory you can contribute to Google movement and I suppose some people do but realistically Google controls it completely and they will do whatever they want. even though the source is available it is effectively not a contribution back.

You can't be stealing what is given away for free. Developers that choose copileft license want to see al changes shared under the same license. Developers that choose permissive want their code to be of use to as many developers as possible for whatever they want to use it for.