No, it's more similar to being denied entry to the US because you have previously driven on the left side of the road in the UK.

Or you have driven at 200mp/h on the Autobarn in Germany.

Obviously perfectly legal in Germany, but seriously illegal in the USA, so the border guards will deny you entry for breaking a US law while you where in a country where it's not illegal! insanity!

If you admit to smoking it, you must have been in possession of it.

I've once found some (something less than a gram), seriously. Nobody was around who it might belong to so I've just smoked it immediately. The law (well, a FAQ on the police website) says I should have taken it and given it up to the police but what if an officer would stop me for a check during my way and find it - wouldn't it be possession anyway? The purpose of possession is unverifiable and forbidding a person to consume any kind of substance is ridiculous.

Why the distinction? Both smoking and being in possession are illegal in the US anyway.

It can make logical sense to ban selling or producing but outlawing a person to consume whatever (except when driving, operating machinery or doing some other kind of job potentially dangerous to others) is plain absurd - every person's body belongs only to themselves and they drink/smoke/eat whatever they want.