Just to be clear, this is not what is happening. Otherwise they could share the accounts and play only during unranked so players who don’t own a bunch of mythics could have fun (I’d certainly love to try a bunch of awakened mythics during unranked).
The difference is that the account sharing is done specifically during ranked pvp which affects all the players who use their own accounts and want to get high rankings. This would fall under the category of gaining a huge advantage in the leaderboards since the actual owner isn’t even playing.