It's not different at all. I consider it cheap, ergo, affordable for parents to pay for it. If the fees were $20k pa then the case for making parents pay for it is greatly reduced as it is a more significant financial burden.
Back to my original point, you said it's "mind boggling" that parents don't pay for their adult child's Uni fees. If a friend told me they weren't going to pay for the child's Uni fees, my mind wouldn't boggle. Each to their own but whether they do or don't I don't think it's "mind boggling'.