Strange, Cyanogenmod always looked pretty open to me.
I never used stock android roms. Since my first Galaxy Ace, I always root them and put another rom. I already used Cyanogenmod, RessurrectionRemix, Paranoid, Mysteryous, SlimBean, AOKP, and a few others I don't recall now.
Cyanogenmod paired with OnePlus makes easy to develop firmware and drivers for it, because nobody needs to get the binary blobs and stitch it on the new ROM, the documentation is clear(er), and the people from CM and OnePlus can talk to each other to solve problems.
Everybody is wanting this phone because of the great hardware specs (on pair with Galaxy Note 4) and a very low price, beating down every single phone on the same price line, and even beating ones costing twice as much as the OnePlus One...