The problem is that people who become prostitutes are often people who have no other choice. So you'll get a bifurcated distribution: people who will only take the job at some incredibly high multiple (because prostitution is really awful compared to other choices) and people who will take the job at any price because the janitors aren't hiring and they need to eat. The point of the comparison is to capture this problem.
True. Ultimately, the problem which requires a solution is not decriminalization of the prostitution but the origin of why people get into the situation where prostitution starts looking as a way out. But, since no one cares, decriminalization is a good thing to do - excluding people from the society for getting into the situation, which is a product of this society, is unfair
