I disagree. IPP is a different service than vanilla HTTP; there's
nothing wrong with having separate default ports for each,
any more than having different default ports for telnet and whois.
(Nobody's required to prevent the use of port 80; it's just that
IPP needs its own default port assigned to it, and the IPP URI
needs to default to that port)
I think this is cleaner overall than using a new HTTP method on port 80.