The reasoning doesn't fall on its face just because you pick an activity that shouldn't be outlawed in the first place.

Interpreting 'should' in the legal sense, then yes holding the bong seller accountable is a valid argument that can be made. And the second scenario is possibly valid too, depending on some details like what they actually sell and what specifically the illegal activity is.

It's not a particularly slippery slope.

I don't really like that kind of enforcement, but it can be done in a reasonable enough manner.