It's a cost increase for students that's not as easily noticed as a "base" price increase in tuition. Which paying more to professors would require. As well as payroll taxes on the salary increase. So getting $1500 directly from publishers the same $1500 from the University costs extra in additional payroll costs (and then gets multiplied by any salary percentage increase in future years as well).
This is also somewhat analogous to what some elementary schools are doing (sorry no citation) with requiring parents to buy school supplies that were previously provided by the schools. It's a way of cost shifting that allows you to pass a tax that isn't recognized as a tax increase.
Other example may be an auto mfg. only raising the price of a new model slightly but removing and making certain features options, options that the majority of buyers will end up adding on anyway. (If the price of the car were higher many people wouldn't enter the showroom at all.)