If I'm in the middle of a huge gun fight I don't care who picks up the tags. But if me and a team mate are say near the edge of a map, both converging on the same enemy and he gets the kill before me. And I see him moving towards the tag I won't steal it. Now if I got an assist and even though my team mate killed him. I will pick up the tag.
The problem is that all Confirm tags are worth 100 points, even if you didnt kill the enemy, change this so that all confirms from a teammates kill are only worth 25 points while giving the player who got the kill 50 points and it wouldnt be such a problem.
So yeah, i agree with the OP, get tired of killing 2-4 enemies only to see some d-bag run up and grab all the tags i worked to get.