It's more conducive to efficient traffic flow to allow you to turn on a green signal by applying the GIVE WAY rule than have only one road on an intersection going at any one time.
Having recently gotten rid of the excellent rule which gave priority to vehicles turning out from a side street over those turning into it (they have two directions of traffic to consider) whilst clearing those turning right into it as a priority over those turning left perhaps this will be the next rule they meddle with and make congestion worse.