I don't think it is that irritating as such. But it's the principle. It much easier to convince people that they should be prudent than at which moments jokes are appropriate. I recently found out that some mplayer developer thought it was funny to include "Core dumped ;)" as the last line when you dump a stream. I'm sure that it was funny to them, but less so when you are just trying to figure out if it works after battling deceptive copyright protection, and learning all the settings of different tools, for a couple of hours. Now try and convince the developers that this, or other jokes, isn't funny.
Man is an UI for people.
Even the man page includes a couple of examples where it isn't.