I have heard it from people I know who are involved in eradicating human trafficking.
Basically, a lot of young people congregate there, and are easy prey for those who would use them for those ends.
Just as people were talking earlier in this thread about the homeless who use the library, it is also considered a good place for kids to go who are trying to get away from an abusive home. It's open late; it's warm; there's something to do. Kids don't have to be accompanied by an adult to be in the library. (After 4 p.m., they must be accompanied by an adult to be at the mall.)
In the summer, there are kids who go to the library simply for meals -- these are the same kids who are on free or reduced-price lunch programs during the school year, who might not otherwise get a meal in the summer. Oftentimes, parents will use the libraries as a babysitter in the summer: they drop off the kids on their way to work, and pick them up at the end of the day.
So, if you're a young girl from a poor family who's going to the library for your only meal, and some dude talks about how pretty you are and offers to buy you something nice, well, you might just take him up on it. Next thing you know, you're in the back of a van on the way to Miami.