Prostitution of minors in France has risen by up 43 percent over the last four years, according to official figures published Friday. The government insists it remains committed to cracking down on exploitation networks, a decade after a landmark law made paying for sexual services a crime.
Sex workers who work legally need to be eligible to work in the country they reside in and they are protected by workers laws.
That makes it even worse. If you stop engaging in prostitution, you get fired and your working visa is revoked. Also, you're assuming all victims of human trafficking are immigrants/don't have the right to work in the country. At least in the UK, unless if you're a student or a tourist, if you don't have a right to work there, you probably don't havw a right to be there either.
Also when sex work is illegal it's mostly done by organised crime and and the right amount of money and leverage employed at the right spots can get the police / the state to look the other way.
Maybe this applies to countries with corruption issues and lack of morals.