I end up wondering if it's just one more case of "want", rather than "need". Unfortunately it tends to be the former, not the latter.
I dunno, if the custodial parent is still paying health insurance, car insurance, cell phone, and is still supplying housing and all that goes with it while the child is in college, I am not sure that I can blame them for wanting the other parent to still contribute. That doesn't mean that legally the other parent has to contribute, just that it can be understandable that the custodial parents wants help and hopes that the law allows for that.
My daughter and the grandkids have moved in and out of my home a couple of times. I have always been amazed at the difference in the bills and the costs of groceries and sundries.