In addition, as far as I know, ba/sh has no equivalent to os.path.join (see, eg: http://unix.stackexchange.com/questions/23208/building-paths-robustly).  So even if we *did* want to "act like the shell" there's no analog to point to.  The shell only expands ~ when it is the first component of a path in a globbing context, and Python does not do globbing automatically anywhere.  So calling expanduser on a path before doing the join allows you to recognize '~'s if you want to support them in your input.  But, as I said, the shell has no real analog for join, so talking about the behavior of join in analogy to the behavior of the shell is pretty meaningless.

The key thing to realize here is that Python does not do globbing automatically (there is a separate glob module for that).  I don't think even a doc update is warranted, since it would be a bit weird to document things we *don't* do :)  However, if you have a specific proposal for what such a note would look like and where it would be placed, we would consider it, as we are always looking to make the docs better.