Actually, they're not. My daughter is in college and in the parent orientation, they told us that any child over 18 can't be discussed. They specifically mentioned alcohol poisoning. Her school made the policy that if the child could talk, then they will follow their direction. If they can't, then they will call the parents. My husband is a college professor and he can't tell parents anything. But again, he has developed ways that he can share info if parents and students come in to talk. It's usually when Johnny knows he's not doing well and it's the professor's fault and he sics parents on him. Smart profs are prepared, though, and can stealthily convey info.
There's a difference between a school's policy and the law. Any school that is eligible to enroll students using government-subsidized financial aid, or that takes any other federal money, is subject to all the federal laws on this topic. But beyond the limitations imposed by those laws, many schools impose further limitations via their own policies. Federal law makes a huge distinction between a student who is legally financially independent, and a student who is claimed by parents as a dependent for tax purposes. For instance, a school can't send parents the grades of an independent student, but can send parents the grades of a dependent student -- age is not the deciding factor (though IIRC, there is an age limit under tax law, for claiming a student as a dependent).