That's thing, there are no official rules for trading merit with your alts (or friends,)
I know it's nitpicking but this (emphasis mine) is what I would consider a "rule" against meriting alts, along with the obvious ethical concerns:
You get merit points when someone sends you some for one of your posts. Additionally, when someone sends you merit points, half of those points can be sent by you to other people.
It's true though that there is no enforcement of that by forum staff, so it's a gray area left to DT to handle.