I have 2 Windows accounts each with Frontier ED (No steam) and 2 Frontier accounts.
Yesterday I bought ED on steam. Created a new account on Frontier and wanted to link the Steam purchase to Frontier. Frontier needs a CD key to link to Steam, but there is no CD key. It actually says so on the steam page. I have no idea why.
........
It changed in April - Steam versions are now non-CD key - so you have to log in via Steam, presumably to stop people avoiding Steam getting their cut from cosmetics purchases.