the idea isn't that everyone needs to be tested before sex. that would be absurd and impossible. the idea is that the women can be tested and can find medical help if necessary, that they have health care benefits, that they can have a safe place to do it, that they can be protected and can work together to build something sustainable instead of living on the fringes of society.
i can't think of a single reason why it should be illegal. people sell their bodies for work all the time. this is just a holdover of a puritan taboo.