It's just less effort all round to have everybody using the same supported set of tools. I don't know if I've ever worked on a project that's gone so far as to actually mandate this, but a "should" (possibly in the RFC sense) is fair enough I think. Every project I've worked on has ended up with everybody eventually settling on using the same tools, because it's just overall less hassle that way.

(Regarding Emacs, I'm typically using it anyway for org-mode, so if I need to do some one-off edit or other - probably some keyboard macro thing - then I can just load the problem file into Emacs and save it back out again afterwards. I've only once been in the situation of neither being allowed to install it nor remotely connect to another machine to use it.)