Same reason why “open container laws” are a thing, where you can be found guilty for a DUI even if you didn’t drink anything, if they find an opened alcoholic beverage in your car.
Or why they can bust you for DUI when you were not driving but just sleeping in your car.
Because it’s one of the tools of law makers and police to criminalize as many people as possible and use selective enforcement to punish certain groups.

Easier for the cops to detain people