TB deployment issue

I had a guildmate deploy in Rote TB to a planet that is already 3*... it was just a mistake on their part, but the larger point is, why does the game allow you to deploy to a zone that is full. Shouldn't the deploy button be greyed out once that zone is full? Seems like this would be a relatively easy thing to fix. We could have used that GP on another zone, and while it will be on the person who made the mistake, couldn't you guys prevent something like this from happening by blocking the deploy button once the planet hits 3 stars?