What's to stop Greece from passing austerity laws, getting the bailout money, then repealing the laws?

There's nothing stopping them from repealing their laws. It doesn't matter anyway because the bailout is only a stopgap measure and we're going to be right back here again before much longer because nothing has been solved. This mess isn't going to go away until the German banks and others holding Greek debt agree to take a haircut on what they hope to receive.