If you actually took your time to go through the rules and follow them then maybe you wouldn't have gotten banned.
I'm not entirely sure but I'm pretty certain that this has always been a rule.
Well because it's exploiting the map. You're not supposed to go into the enemy spawn for a reason which is because spies can basically spawnkill people in there non-stop and your team can't do anything about your teleporters. Letting friendly spies in is still not allowed as it would most likely later on lead to the spies killing people anyways. Also, even if you let friendly spies in there's surely going to be a player on the enemy team which will abuse your teleporters just to kill your teammates.