With prostitution, it makes sense (if you want to criminalize it at all).
You don't want to criminalize people who are often pressured into prostitution, because that would cut them off from getting help or going to the police if they are abused.

With weed, what the Dutch model tries to achieve is to not punish people for smoking weed, but also not turn it into a for-profit industry that would create an incentive to get more people addicted.
I think the German model is better in this regard: Let supply also be legal, but non-commercial. But they went overboard with the regulations (police can get a list of all consumers at any time, without a court order or criminal suspicion). And it looks like the decriminalization will fail at the last minute anyway.