The sex trafficker can absolutely be tracked by doing old-fashioned police work: you spend time, money and energy to infiltrate the network, gain their trust and eventually take them down. But this requires police funding and training.
“if you’re doing nothing wrong, then why do you need to hide it?” An age-old dilemma.
It’s not a dilemma, the answer has been given multiple times: under the rule of law, law enforcement has to prove (or at least demonstrate a strong suspiscion) that you’re involved in illegal activities before they can intrude in your privacy.
But with the advent of mass data gathering and the exemple given by the NSA, all law enforcement agencies dream to change this paradigm.