Why do I get a loss when the opponent forfeits the game?

I have played two games now where the other guy leaves and I and rewarded with a lose. The one I was losing 1-0, then he left and I get a loss. The other game the guy left 5mins in while it was 0-0, and it counts as a loss towards me and I lose a entry for the legendary legacy tournament. I just don't get it, I get a loss even though the other guy forfeited the game?