the point is that the legality is determined by the law enforcement officer who specifically creates the illegal scenario by bringing up drugs. it is their speech and actions that create another man’s crime and then arrest him for it. do you really want the police to have such power over people?
There. Fixed it. You are trying to paint the scenario that Law Enforcement gets to determine illegality. That is inaccurate. What determines illegality, in this case, is the the person who buys the bong articulating his motive to use the bong for drugs.
For example, you could walk into the store, just an ordinary Joe Bloggs, and say "I want to buy a bong to smoke some marijuana." BANG! The law has been broken once the store owner sells it to you on that basis. You intend to use the bong for an illegal purpose.
If you were to go to a fertilizer store and ask to buy some Potassium Chlorate "so that I can make a bomb to blow up a school" the law will similarly have been broken if the store owner sells it to you, knowing you are going to make a bomb with it.
If you were to go to a gun shop and ask to buy some bullets and a gun "so that I can assassinate a head of state" the law will similarly have been broken if the store owner sells them to you, knowing that you intend to murder someone with them.
This is exactly how it should be. If the store owner knows you are going to do something illegal with your purchase, he is culpable if he sells it to you on that basis. That is how the Common Law works.