The thing is, those dealers can end up in jail for selling drugs.

More to the point, if a kid walked into a convenience store and the clerk sold them a pack of cigarettes, the clerk wouldn't get off the hook by claiming, "well, the parents are responsible for their kids." I'm also not sure how one would justify holding parents legally liable for crimes they played no role in committing.

I'm not saying that I agree with these laws. They appear to be taking things too far. But that has more to do with there being no clear way to define sites that are only of interest to adults (no gatekeeping needed) and sites that should be restricted to adults.