Nothing at all. However, what is to stop Apple from reading every single one of your personal files on your OS X Mac (or MS with Windows)? Again, nothing at all. At some point, you have to trust that your OS vendor is not out to get you. Or, if you lose that trust, you have to switch vendors. Or, if you are truly paranoid, go to linuxfromscratch.org, review a few MBs of source code, and compile your own OS.
