This may be true, but it’s the best measuring that we have. I don’t know if it’s worth throwing out the existing measures we have from before and after legalization on a hunch that it might be different results. The surveys try to account for this as much as they can.
I agree that people shouldn’t be punished for their drug use, but I think the point is that without punishment, drug use increases. And there are some negative impact from that increase.