You're comparing a street vendor to a prostitute? What are they? Entrepreneurs? You think they're going to go to city hall and get their prostitute licence? Check the zoning laws?
Prostitutes are victims of desperation in society... dispite what they may tell you, they don't all love having intercourse with every person willing to give them money for it. They put themselves at risk of disease, rape, robbery, and pregnancy because they need the money to support themselves better than what they feel a normal job can offer them.
...and for every guy who goes to a prostitute thinking his the toast of the town and treats all his girls with respect, there are five that just want to get a quickie with a girl they would normally never have a chance with. They are with no respect for the girl and are capable of providing her with all of the risks listed above. So please... don't compare these girls (or guys) to hotdog vendors.