Huh, I guess I've never given it too much thought. I was under the impression for some reason that the first argument was supposed to be an absolute path.

It might have had to be absolute on ancient Unixen ... Unices? Seems POSIX has all of this to say about shebangs:

    If the first line of a file of shell commands starts with the characters "#!", the results are unspecified.

So it's basically all down to convention, but one that's been followed long enough that you can rely on it. I still don't count on shebang taking more than one argument to the command though.