How do you know all this? Do you have access to their records?
What do you mean that interpreters don't even work half the time? Do you mean that they don't show up, or that they sit on their hands when they're supposed to be signing?
Some colleges require sign-in sheets for the terps. Each class, each meeting, the instructor signs and dates, and the terp signs and dates, to prove that the terp was present.
I don't know how other schools operate but it would be impossible for me to "scam" the school. For one thing, if I don't toe the line, the students are not afraid to complain to the Disabilities Office, and I guarantee you that I would hear about it from the boss. (It never happened to me but it has happened to others.)
If anything, I don't even charge for staying a little overtime for some classes, or showing up a little early so the student can have a few private moments with the instructor, or meet with project team members for discussion.