That doesn't mean they work with ALL cover songs though. It's similar with gaming channels in that even if I join a network and they protect me from content ID claims, they still have to follow copyright laws like anyone else on YouTube, by me having copyrighted content that I didn't have permission to post, I become a liability to the whole network, including it's partners. The reason for this is because if a developer/publisher who doesn't like videos of their games being made or likes claiming their own content sees my videos despite being protected from content ID claims they can still initiate a DMCA takedown on my video, this results in a strike for my channel and from what I recall damages the network because other people on that network can't monetise their videos if someone on the network has strikes, anyone can feel free to correct me on that 1 but the general idea still sits, you're a liability when copyrights come into play. You also have to factor in YouTube's own policies which is basically that you need 1 of 2 things to post content. 1) you need to own the copyrights or 2) you need to have permission to post it if you don't own the copyrights.