No one said anything about playing poorly.

Think of it this way. Say the tank, knowing their own limits, knows that they can safely pull (and hold) ten mobs, but no more. So they gather those 10, and proceed to do their job.

A DPS decides that 10 isn't enough, and purposely pulls 5 more. The tank (going by your theory) pulls them all off of the dps. However those 15 mobs are a bit beyond the tanks ability. So the tank starts taking damage faster than it can be healed. The healer begins to struggle, and the tank eventually dies. Then the healer, then the dps. Congrats, we have a wipe.

If the tank simply let the dps die, all the mobs would then move to the tank. Maybe they can pull it off, maybe they'll still wipe. But at that point the fault would be on the dps, who would hopefully learn their lesson.

So really that would be the dps playing poorly. A good tank knows their limits, and will pull accordingly. They set the pace. And the dps should follow.