RDM: Not allowed
No-proof: Allowed.
What the difference is is this:
RDM is killing without ANY reason whatsoever.
No-proof is killing someone you believe is traitor, even without solid proof.
There is a big problem with that reasoning, IMO. People can then RDM and claim they believed he was traitor with no proof at all, if what you say is true then they'd get away with it too since after all, RDMing isn't allowed, but killing a suspect without proof is.