There is no "shared ground." Prostitution is illegal in all but one state of the USA, and that under very specific conditions. Craigslist should self police and remove posts that more or less are openly illegal in a way defined by the laws of the land. It's not like they are just discussing it or using free speech, its openly advocating the sale of illegal services and should be against Craiglist's own TOS.
I mean hell, if they were facilitating the illegal sale of guns this site would be up in arms to ban them.
In general the public does not treat all forms of illegal activity facilitation the same and this is a good thing. Some forms of illegal activity facilitation are much worse than others in the eyes of the public. It’s reasonabke and natural that one instance of this evokes outrage and another a nonchalant attitude.
I mean hell, if they were facilitating the illegal sale of guns this site would be up in arms to ban them.