It's been that way for a very long time, because it's based on the US definition of "commercial item".
I know that the 2009 DoD policy on OSS attachment 2 part 2 spwcifically states this. It said, “In almost all cases, OSS meets the definition of ‘commercial computer software’ and shall be given appropriate statutory preference in accordance with 10 USC 2377 (reference (b)) (see also FAR 2.101(b), 12.000, 12.101 (reference (c)); and DFARS 212.212, and 252.227-7014(a)(1) (reference (d))).” The most recent version of this policy says the same thing.
I know that the 2009 DoD policy on OSS attachment 2 part 2 spwcifically states this. It said, “In almost all cases, OSS meets the definition of ‘commercial computer software’ and shall be given appropriate statutory preference in accordance with 10 USC 2377 (reference (b)) (see also FAR 2.101(b), 12.000, 12.101 (reference (c)); and DFARS 212.212, and 252.227-7014(a)(1) (reference (d))).” The most recent version of this policy says the same thing.