Pretty big discussion thread about that here if you want to review some posts players made, I’m quite sure that it’s on Blizzard’s radar its just that they don’t want to take action for an at-will activity since you can form the group and vet them:
To inform: Players who AFK or leave during a mythic key are not in current violation of any rules and will not be actioned against. Unless: they intentionally write in chat that they joined to and did ruin the key.