If they do review the source code?
Even if DHS (or whoever) reviews the source code, that's no guarantee that what's installed was built from that source code. In order for the source code review to be meaningful, DHS would have to build the executables and installers from that source code and create an internal distribution. They would also have to review and host all virus pattern definition files (or whatever) that are pushed to the PCs periodically.
A project like that undertaken by the gubmint would take many years and cost thousands of lives....