Because at some point, the $9.50/hr job at Taco Bell will run its course, and your child will end up with severely limited resources. Together with a lack of funding and poor health care options, your children will probably return back home to "evaluate their situation". At that point, they will start wondering why they have been left behind in society, why the system is rigged against them, etc.
As we all know (and frequently discussed here on HN), a minimal wage job results in limited housing options, limited healthcare options, limited educational options, etc. As a parent, it is my responsibility to prepare my children to become self sufficient beyond the minimum wage job. For the sake of themselves and their children.