The shebang line has never been specified as part of POSIX, SUS, LSB or any other specification. AFAIK, it hasn't even been properly documented.
There is a rough consensus about what it does: take everything between the ! and the \n and exec it. The assumption is that everything between the ! and the \n is a full absolute path to the interpreter. There is no consensus about what happens if it contains whitespace.
1 Some operating systems simply treat the entire thing as the path. After all, in most operating systems, whitespace or dashes are legal in a path.
2 Some operating systems split at whitespace and treat the first part as the path to the interpreter and the rest as individual arguments.
3 Some operating systems split at the first whitespace and treat the front part as the path to the interpeter and the rest as a single argument (which is what you are seeing).
4 Some even don't support shebang lines at all.
Thankfully, 1. and 4. seem to have died out, but 3. is pretty widespread, so you simply cannot rely on being able to pass more than one argument.