If you're somewhere prostitution is illegal, I can almost see it as 'if we don't have people known as prostitutes, then we've a higher change of getting away with this'. (See the related way Anika Mae was banned from stripping at a local club once they found out about her escorting, for example. Mammothly unfair, especially given escorting is legal here, but it would have made their licensing easier.)
More likely it's 'if we don't have people with prostitution know-how, then we've a higher chance of getting away with stuff with them', so yes, warning bell until proved otherwise.