It was released as freeware back when by EA originally, which is what openRA relies on. Later the source was released under GPL[1] during the CNC remastered collection from EA.
I think you're right - it encouraged sharing, but usually limited functionality (use) unless you knew the secret code "TIASP1814" - sorry scorched earth 1.2, you didn't give me a free upgrade 20 years ago, and I've been holding that grudge forever.
[1]https://github.com/electronicarts/CnC_Remastered_Collection/...