It’s done for smurfing (“derank”) purposes, as in teamgames just leaving the game right away doesn’t ensures defeat. So that’s one way of making sure some MMR is lost.
Occasionally, some raging users might do it to punish an ally they hate/have a disagreement with ; but that’ll be specifically against some users. If it’s done indiscriminately, that’s just simple smurfing.
More info about teamgames smurfing here, in part V (just CTRL+F twice and type “V - teamgames” ).
Freelosing smurfs aren’t punished either, and they’re many more than teamkillers. The devs are yet to be convinced that this is an issue I’d say. 