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.