IANAL but I think the license you buy from google play or apple or whatever is not a license for the content, it's a license to access the content from the provider through a specific player/app. So getting content from musicbackups dot com or whatever would not be a transferable right or gray area.
Not sure about US, but for example in Czechia, you can download any content legally - as long as you don't share it, it's fine. Torrents do share it though, so that is a gray area.
You also have a right to make a personal backup/copy of the content, again, as long as you don't share that copy (projecting for multiple people is also sharing in this context).
People will do it anyway though