However, that doesn't necessarily mean they're public domain. Trademark law still applies, for example.
Additionally, not everything the government uses, even exclusively, was produced or is owned by the government. Often, government contracts allow the contractor to retain control and ownership of the intellectual property. The government may also have copyright transferred to it and retain that copyright. A legal issue I haven't researched is the line between a work that is a government work under work-for-hire principles, and therefore is ineligible for copyright protection, and a work for which the government contracts and for which copyright is subsequently transferred.
Before attempting to FOIA the source code of a piece of government-exclusive software, I would first FOIA all government contracts for the creation of that software. Then you'll have something to go on when crafting the FOIA request you really want.
https://www.irs.gov/privacy-disclosure/use-of-federal-tax-in...
lots of vague stuff, but looking at the Google result, I noticed some differences. Ah, the meta description of the page reads:
Open source software, while it can be useful in many instances and appear to be cost effective, may present a security risk because open source developers don’t typically follow security best practices when developing their software.
Well, there ya have it! :D
https://www.irs.gov/e-file-providers/modernized-e-file-mef-u...
Because many eyes often make bugs shallow, there is a pretty good chance that a public release of the code will find errors that both find funds that are owed to the government and exonerate people who have been incorrectly billed.
What better purpose for the American Fuzzy Lop?