There's a whole lot of evidence that prohibition vs legalization each have negative effects and it is up to society to decide which is more appropriate in certain situations.
I doubt that food regulations are going to result in black market Doritos barons waging deadly turf wars in the streets. But I suspect it will result in food manufactures building a much worse product in response.
Another way of writing that headline is potentially "cannabis users 24% more likely to admit to using cannabis when in a state where it's legal", or even "marijuana legalization brings cannabis users from out of state".
Even if the study you're referring to does effectively control for those two factors- I would hesitate to apply those results to a harder drug like heroine. Marijuana education and legalization are happening at the same time. The information on Marijuana is pretty promising and education may incentivize it's use for some purposes. I do not believe the same about heroine- education would probably disincentivize it's use.
This may be fine enough for weed, but what if harder drugs start being used 24% more? Even if they're in more pure form than from illegal sources, that doesn't sound like a net positive.