You can't traffick humans without kidnapping them and holding them against their will. The whole point is we already have laws on the books to prevent that regardless of whether prostitution is legal or not.
In the case of people most jurisdictions want more severe laws in place than possession of stole goods against those people who are holding others against their will but did not kidnap them, as in they received the victims from others who may or may not have taken them by force.
This is not important. That sort of slavery is easy to deam legal without getting into what the forced work is.
Sex work = trafficking is, best case, a misplaced utopianism that makes things worse here and now, and often a far worse patronizing moralistic sexism.
