I think it's the fact that when students know their parents are paying for their college tuition, they can do whatever they like, knowing they'll be covered by their parents.
I once heard a story from someone during the college about a parent found out that their son was failing the semester because he was playing and partying around. They had a talk with their son when he came back to home. The parents told their son that they would no long continue to pay for his college tuition and the son would have to cover all the fee by himself if he want to continue attending the college. The son was shocked and begged his parents not to do that. The parents decided to give their son one more chance but they set down the rules. The parents want results, a good grade and GPA. If the son could give his parents that, they would continue to support him and pay the college tuition fees. This was a huge slap in the face for the son and from there, the son took college serious and his grade was improved by huge margin.
I think that is kind of the situation that all students need to have. The students need to understand that they have to grow up and accept the responsibilities the moment they graduate from high school. College isn't high school, it's a whole new world, one that comes with consequences. If every parents continue to spoil their children by continue to support their college tuition no matter what, their son/daughter will probably be likely to take advantage of that.