This article covers why it’s criminal: https://www.nytimes.com/2019/03/14/business/dealbook/college-admissions-bribes.html
This passage kind of sums it up: “Mail and wire fraud statutes identify a scheme to defraud as including the “right of honest services.” That turns the dishonesty of getting your child admitted to their college of choice into a crime that is punishable by up to 20 years in prison.
The honest-services law had been a means to police corporate or official dishonesty without requiring proof of a benefit to the defendant. But in 2010, the Supreme Court, in the appeal of former Enron chief executive Jeffrey Skilling, limited honest-services fraud to cases involving bribes and kickbacks.
Now if a university coach or employee takes payment in exchange for improperly admitting a student who is not otherwise qualified, it can be a federal crime because it violates the honest services owed to the school. For the parents charged in the case, paying the bribe means that they are just as guilty for acting as an accomplice in the fraudulent scheme.”
This all differs from benefitting the entire school by donating a substantial enough money to fund facilities that will benefit all.