A few years ago a reporter did a story about this and found that a not insifnificant number of these kids are from well to do families up north who come down here to play homeless during breaks from college and in the summers. They treat it like backpacking through the wilderness or hitchiking cross-country. Whether that has changed, I don't know. But, I doubt it has.
And why do you think is so complex about this issue? They are loitering, comitting crimes, and intimidating people. Regardless of where they come from or the circumstances that let them to be here, it's something that needs to be taken care of.
Beyond that, they are over burdening programs for the truly homeless that are underfunded to begin with and make is trully difficult to tell the difference between the truly homeless that need help and those who do this by choice and because it give them the freedom that they want.