Chances are they're using something like Tunecore where you sign up once, and they redistribute to various music services. You still retain whatever rights you have, but you collect your income from Tunecore after they aggregate it from Apple, Amazon, Spotify, et al. IIRC, Tunecore does not take a percentage of each sale, but has a yearly fee to keep your music listed using their services.

I imagine it's because there is a non-zero cost to hosting music, even if that music is never played by anyone. It also discourages people from spamming the store with junk.

you pay them zero, but from the point of view of TuneCore it costs money to store and deliver your content, account to you, run a website, etc.