Every single raiding guild I've been in has had a no chatting in trade unless it's for buying/selling, and no trolling, harrassment or abuse on any channel although general chit chat and joking around was fine (but not on trade).
I am fine with a guild having that sort of rule, because I've never had the desire to contribute to the misuse of the trade channel, nor to troll or offend anyone in the game. Comes down to a choice though, as long as you know before joining the guild what the rules are I really don't see why it would be a big deal either way. Either you like a guilds policies or you don't.