Omitted attribution is of course a license violation, but their issues are not limited to just that case, and it's the other issues I'm addressing.

If they didn't even give attribution, why would we think they'd care about copyleft at all?

In the permissive world, nobody cares about attribution. Many people actually don't even know that "permissive" doesn't mean "public domain".

In the copyleft world, people are a lot more vocal. If you abuse a copyleft licence, chances are that some people will make noise :-).