You're still taking up space that could be used for a player actually playing the game, or causing unnecessary server load. We all share the game. What insult is it to be removed when not using it?

If it's that much of a bother to you, you can use one of those anti afk kick macros you where talking about. Really, its not even an annoyance. If you're gone from you computer that long, what's 30 seconds of logging back in? I don't see how the extremely minor negative outweighs the minor positive.