I used to think that way, but I've learned that there's one good reason for why ...

I used to think that way, but I've learned that there's one good reason for why the API is designed that way: priority inheritance. Priorities are bound to threads, and, when a high priority threads wants to lock a currently occupied mutex, we want to bump the priority of the current holder. And POSIX requirement makes that easy --- the thread who locked the mutex must be the one holding it.