Same old complaint posted a thousand times. I'm not saying you're wrong at all. Just saying this has always been an inconclusive discussion.
I believe when a player is reported and flagged afk a number of times that they do get investigated by a GM.
It is a reportable offense although it is very difficult to prove without a lot of evidence. You'd need a number of screenshots. Video would probably be ideal. The main challenge being that it's a lot of work to report one person so naturally people don't take the time to do it.
Suggestions have been made to intensify the anti-afk system banning people after a certain amount of flags. Having a deserter debuff or extended time they can't queue again. But it's just something that a group of players could use to harass someone.
It's a similar issue as ninjas in PvE. It's just kinda something you should expect going into these random match up environments.