Funded Grad students -- by contract -- are not allowed to moonlight at all.
Yet they do. I know I waited tables while I also put in 80 hours a week as a grad student, and I served a table of my employers! They obviously didn't say a word. But it was in my contract.
Prepare to be astounded! Universities take a huge cut of research money for general use. Now, while universities take 60% of the grant and the rest is given to the grant winner(s), it's the university's job to fund all indirect costs (buildings, facilities, supplies, etc.) which means that the university then returns about 30% of the grant to the project. The rest goes into the general fund. Effectively, it's 30%.