The game doesn't encourage that - it's just the trap a lot of players fall into. "Hey, I've got the boss on me, you expect me to run over and pick up other things, too? You're not taking much damage from them - you can handle them no prob."

Lowbie examples being that Knight Hest where skeletons occasionally spawn in twos. Given the challenge ends when the main guy dies, most just nuke him while the tank holds it and everyone ignores adds. If the skeletons were actually posing any kind of threat to the non-tanks this way, tank players would be forced to reconsider their method very early in their career, but alas, SE don't want Hests or Novice Hall tasks to be overly taxing on the novices, either. If a player falls into a static-tanking mindset as a result of ease of content, then it's on them - I don't think any player worth their weight in digital salt would actually believe it's the way the game expects them to tank down the road, especially not when even low level content starts picking up the difficulty pace at about level 40+. Any sensible tank would see free mobs hitting their team and immediately think "Hey, maybe I should stop this shenanigans". If they don't, they shouldn't be tanking to begin with.