The reason I ask is that the term "open source" was designed to be more palatable to businesses. In Revolution OS, Eric S. Raymond said in regard to saying open source over free software: "If you walk in to an executive's office and say 'Free Software', OK, If you're lucky, the response you'll get is something like, 'hmm, hmm, Free Software, must be cheap, shoddy, worthless.' If you're not lucky, it has associations with the Free Software Foundation's wholesale attack on intellectual property rights, which regardless of what you think about the ethics of that, it's lousy marketing, it's not something that businesses want to hear" [2].
I don't know how savvy the IRS was about the nuances between open source and free software (probably not very), but if they were, they might be making a somewhat-reasonable distinction that "open source" was designed specifically to please the business sector rather than free software's focus on ethics and freedom.
[1]: http://democrats.waysandmeans.house.gov/press-release/new-ir... [2]: http://www.cswap.com/2001/Revolution_OS/cap/en/25fps/a/00_54