Actually RMT does have a lot to do with why things are how they are, so it's reasonable why people include that to their argument. Mind you, RMT is often also associated with exploits, hacks, and bots (cheaters in general). This is not a new concept, and all MMORPGs tend to consider them as a factor in some way (at least all of them that bother to take action against RMT activity). Especially given SE's personal experience with RMT from FFXI. Their presence was significantly felt and they OWNED practically everything to do with FFXI at the time. All that endgame content you wanted to do? You're competing with RMT/botters/hackers because it was very easy to monopolize in that game. If you make it too easy for RMT to act (which is what would happen with a lot of suggestions by players), they will take advantage of it.
RMT most likely wasn't part of the reason at the time of the decision to disable /tell in dungeons, since this was done before live (before the resulting /tell spams and whatnot). That being said though, because of that decision, those of us that don't care about the lack of /tell in dungeons don't have to continue to be harassed by them in areas that the chat window might be a little important (party members saying things, etc). It's an unintended benefit from a somewhat unliked, and intended, decision. Icing on a questionable cake sort of thought, I guess. Obviously some things, like the /tell, don't have such drastic effects like what I mentioned, but as I said, RMT was not a factor to their decision to disable it.
Edit: And just to clarify that statement about making it into a big deal...
Statements like this make me think people take this disabled function too seriously. Of course, I'm not talking about foaming at the mouth serious, but just that you folks are making this out to be a far bigger problem than it really is. That the dungeon time spent suddenly ruins the game because a conversation with people you consider friends, folks who will be there after (or next time they log in), was temporarily cut short... all righty then.