That thread is certainly not a good example of communication, but I cannot find any place where a representative from the Muse group says that they own the copy

That thread is certainly not a good example of communication, but I cannot find any place where a representative from the Muse group says that they own the copyright to all sheet music on their site. They say (and that's to the best of my knowledge mostly correct at least here in Germany) that the arrangement for music is legally protected and even if someone else typed down the sheet, the rights to this sheet are still protected. And they point to the music industry enforcing restrictions (having worked with the music industry, I find that one entirely credible)