College is an education of the mind, but that expands far beyond the classroom. This is the first time many people have the opportunity to deal with life without a safety net, make their own decisions fully, etc.
If a parent wants to control their child's life past age 18, make them stay at home. If you let them go to college, then you're admitting that you feel they're enough of an adult to make their own decisions, and the second a parent says they'll pay for the child's formal education, then what the child does outside of obtaining the formal education becomes a non-factor. If their grades plummet, then it's their responsibility to pull them up. If they make bad choices outside of class, then that's their responsibility. Cutting off the money for the formal education does nothing but either make a bad situation worse, or make a good student have to leave school due to something totally unrelated.