One way to approach tax design is like bridge building. Anyone can do it. Throw a plank over a creek and you’ve got yourself a bridge. Say marijuana consumers pay an extra X percent at the retail counter and you’ve got yourself a tax.
At a primitive level, that all works. For a while. But the plank won’t handle much traffic. And with the marijuana tax, what happens when market forces drive prices (and taxes) down? What about a free pot with pipe purchase? And employee discounts? And pilfered product — or product that’s “lost” in transit? The way we tax alcohol and tobacco is not vulnerable to these problems. More here: https://newrevenue.org/2015/04/28/weight-base-arbitrary/.