Alright, did some testing and found out that it is involved with the .exe.
I ran "arma2RFT.exe" which is the pirated Reinforcements, and FADE started. Then I ran "arma2oa.exe" which is the legit one I paid for/ key and all that.
I think you can use a single key many times. If so, I could probably find a key that no one cares to share, use it in the installer, and use RFT for singleplayer only.