Before th first wave even arrives, players are given infinite amounts of time to build and upgrade towers, set rally points, and move their hero. So, why should they have to restart just for building one tower that they didn't want to, or upgrading something by accident?
Tower should sell for their full price to get all of the gold back after being built and upgraded so that players won't have to rebuild their entire setup. This is especially crucial to players if they're like me and want to stack 3+ barracks over each other and need them to be exactly in the same spot.
Though minor, I feel that this is an addition that would show KR as being that much more refined and perfect amongst its competition.