Think about it this way, if you have idle people booted, you will have, let's say, 12 people in the channel on average. Anyone new who comes in and sees 12 people in the channel will go, this game is fucking dead (more than it is).
on the other hand, seeing 100's of people who never talk or play and just, well, sit there, also creates the impression of a "ghost town". Personally I'd rather see 5 active, playing/talking/alive people rather than 30 forever-afk bots. I think it's not a binary issue here.
Also, I often idle while eating/watching something and go back to see who is on or what messages I've missed. This enables me to want to play more if I've missed something or someone is on or says they will be back in 30mins.
Well, there's /away command too. The "brb 20 mins" affecting your gameplay has literally less than 0.05% probability of happening, why even include it in this post?
It doesn't harm anyone and it has no adverse affects on anything. Why make a rule for no reason?
it's harder to find a game, one can never tell if anyone's in the channel, if I saw only myself i wouldn't bother with "1v1 wanted" messages, which may happen in a channel with 15 idle players/bots.
moreover, it ruins the "/who war2bne" command which can be used FROM a game to check who's online to (possibly) invite a certain player to the game. Because the screen gets filled with worthless perm-idle bots.