You seem to have answered you own question.

"... when referring to the accounts any user has on a ..."

One user. Multiple accounts. "User accounts". "Users Account," while not meaning anything at all, sounds like multiple users are using one acocunt--or like "Brothers Grimm..." but that doesn't work in the same way...

You seem to have answered you own question.

"... when referring to the accounts any user has on a ..."

One user. Multiple accounts. "User accounts". "Users Account," while not meaning anything at all, sounds like multiple users are using one account—or like "Brothers Grimm..." but that doesn't work in the same way.