This is answered by the PEP:
A partial output implementation, where strings passed to fp.write() would be converted to use fp.newlines as their line terminated but all other output would not is far too surprising, in my view.
Because there is no output support for universal newlines there is also no support for a mode "rU+": the surprise factor of the previous paragraph would hold to an even stronger degree.
I've read that, but I don't agree with the second paragraph. Universal newline support is available only for input. This sentence is enough to easily predict the behavior in every usage case, including "+" modes.
If there's any intent to add support to output, then I'd understand the exclusion, since this would make backwards compatibility possible (users of "rU+" could have their code broken, since they were doing output by hand). Otherwise, it should be allowed, IMO.