The big thing about going AFK is that it takes up a server slot. Those spectator slots are to be mainly used by admins and not for other players to sit for extensive amounts of time or to use it to switch to a preferred team from one they were previously on as an example. If you need to leave your computer for whatever reason (getting a drink, bathroom, etc.), it's preferred to just disconnect and rejoin in when you're back. Being a GC member, it's not like you can't just rejoin the game when you reconnect. As you stated, it wasn't even full so disconnecting wouldn't have affected anything. Spectator slots are a special case and should only really be used by admins.