Oh, but you can and it has been done. The repeal of DOMA was the first time the US Supreme Court has actually addressed this issue and said that laws cannot be based solely on morality. However, as long as you have morality + something else, you can have laws that would criminalize prostitution. I am mostly with you on this issue, but the details of all of this are much more complex than "let's just make it all legal".
Oh, but you can and it has been done. The repeal of DOMA was the first time the US Supreme Court has actually addressed this issue and said that laws cannot be based solely on morality. However, as long as you have morality + something else, you can have laws that would criminalize prostitution. I am mostly with you on this issue, but the details of all of this are much more complex than "let's just make it all legal".