It's all about enmity control and balance!!!
This is one of the features I really like about this game.

If an overzealous DPS or Healer wants to take enmity away from the tank there is little a tank can do.
It's a balancing act between doing as much damage/healing as you can do without having the mob attack you.

The party as a whole is responsible for controlling the enmity of the mob, not just the tank.