I had no idea os.path.join worked this way and now I'm terrified how many bugs I've left behind over the years 
It makes sense when you think of /bin being a path with a root and not a component that you add and which comes with a separator for padding.
I just wonder how obvious that is going to be for users of the API. Maybe throwing if you append a rooted path would be safer?
Edit: If it behaves as it does, mirroring Python's name join might be better than append.