OSS != FOSS
An AC made a comment about it being better to use OS because it's free. However, software being open-source and software being free are two entirely separate things. I can release the source to my app but require you pay to use it, and I can make my app free to use but not share the source code.
In fact, a lot of OSS software would not be usable by the government for free. Anything under GPL, for example, would require the government to open-source their projects which is unlikely to happen. In such cases they'd have to arrange a separate license which would normally not be free.