Some people offer things for sale which they don't have any right to sell. That doesn't imply that those who do have that right should be prevented from exercising it just to make things easier for law enforcement. You do more direct harm by unjustly prohibiting the activity than you stand to (possibly) prevent.
Further, stealing is usually considered less severe than rape. At least people can take measures to prevent theft.
I don't know. How do you prove that any paying job between a paying company and their employees aren't coerced and that the company isn't abusing their employees? As seen with some recent lawsuits, you don't for years until something eventually prompts investigation.
I imagine it's the same here. If it's illegal then the workers have no legal resource and are trapped until someone discovers the company (if ever. Many times these "companies" are just private individuals. i.e. pimps). If it's legal there's at least some chance the worker than defend themself against abuse if contracts are breeched.
You can't/don't, but regular jobs are not usually associated with trafficking. And three tends to be less privacy - people don't care as much to file their brothel visits as their cafe visits - wrt regular business, the IRS is known to be formidable if all activity is known about.
> If it's illegal then the workers have no legal resource
prostitution is the job many locals don't want. A similar situation might arise in say, illegals trafficked in and having their passport taken, but this is considered less of an issue do to competition from non-coerced labour, and that labourers are harder to threaten by single "pimps".
How about the argument that if prostitution is legal, it's possible for an illegally coercive (trafficked) business to pose as legitimate; whereas if prostitution is not legal, any appearance of prostitution can warrant scrutiny.