I'm reading about this a lot lately and I'm not sure if it is perfectly legal or not.

I mean... It is their own money, so they should be able to do with it whatever they want, but sometimes the smurfs break the game, so...

Does anyone know if Blizzard has an "official" opinion on this?

Having a second account you play on is fine. New accounts very rapidly adjust to play at the proper skill level.

Doing anything to manipulate your internal MMR or Skill Rating (i.e. Boosting or Throwing) is not fine.

Penalties for boosting and throwing are about to increase dramatically. Please report the behavior (reporting is being worked on for Console but is not ready yet).