Yea, they really should do that, make it a loss but don't lose a promo slot. for as far as I know they haven't done that for 2 reasons tho:
1: you could premade with a person and have him leave if you think if you lost lane.
I have actually seen this; a guy that played with an afk and then tried to invade, if it failed he remakes. They could simply fix this by letting the player still lose promo's if a guy is afk that the player is premade with.
2: you could troll your teammates to make them want to leave.
Imagine if you are losing in your promo's. If you really want to ignore this loss you can get your teammates to rage quit by being a huge ass.