One last time. If you don't get it this time, I don't know what to say.
In a YM Crew, if the Tank enters a room, let's say he aggroes 5 mobs. There's a total of 7 in the entire room. Standing off to one side, and building hate against the 5, the crew begins to kill the mobs. They deliberately only kill the 5 mobs on the Tank, and leave the unaggroed ones alone for the time being.
Now, assuming enough time has passed, the Tank has built a lot of hate points. The crew begins to attack the 6th enemy in the room. By your logic, regardless of how many hate points the Tank has, the mob immediately begins attacking the crew instead.
But, what would actually happen, is that the enemy does not attack. Instead, it remains passive, while the DPSers attack, and kill it. All the while, the Tank is still building hate points from heals, far away from the enemy mobs.
Does that make things clear?