It's a middle ground similar to the handling of pornography (often legal) and prostitution (often illegal) and it doesn't make much sense. Usually production, trafficking, and sale are also considered illegal, because to only make illegal the latter would make it more much difficult to prosecute those involved in sales and distribution. Some users need more than others, and some users produce their own supplies. Neither should be illegal, but both often are.

Either it's okay for adults to consent to activities that don't harm others or its not. People should have the right to choose what to do with their bodies, and the legal system should have to ability to disincentivize antisocial behavior in ways that are productive i.e. that actually reduce it.

The antisocial behavior is not people selling/smoking a joint or having/selling consensual sex -- and while it might be associated with them, the government should be tackling those other, actual problems instead.