You know, once upon a time we tanks were lazy. A bard or a thief would run off into the great beyond and come back with a monster hot on her tail. We'd provoke it off her, and the party would go to town.

Of course, things change and now we are in a modern mmo dungeon, but whenever someone runs off and pulls some mobs back I remember those days fondly and just pull them off. You can try to "Teach Lessons" all you like, but the fact remains that monsters aren't very dangerous in lower level dungeons (and even some higher level ones) and DPS aren't as squishy as they might be in other games. Plus sometimes healers like a little challenge and spamming Cure II isn't what I would call a challenge. So that's why you'll see them healing the rest of the party members - nowhere in the healer manual doth it say thou shalt heal onry the tank.

That behavior doesn't generally carry over into serious raids.