Im certain there are plenty of ways to implement it. Just something or someone is stopping them from doing it.
There is no technical reason for not being able to port their server code to Windows/Mac/Linux, assuming it doesn't even already run on one of these operating systems. As you have rightly guessed it is a political/financial/DRM/anti-cheating/protecting IP/etc decision.
Worst case they'd have to remove a lot of encryption, authentication and possibly database stuff, but the basic network architecture could stay the same, just like has been done in one hundred+ other games.
Then the client and server could sit on the same machine, or you could run the server on a dedicated box and have it hosting your own local private LAN, once again as has been done in one hundred+ other games.
My guess is that this client-side server code has been developed in parallel for the entire development cycle and was pulled very late in the process. FD have been remarkably transparent in all of their development process and I find it hard to stomach that they have known for months, or even longer, that there would be no offline mode.
Why it was pulled, (assuming my guess is accurate), could be for any number of reasons, (see above), or even just as they said, it became financially/resource nonviable to maintain.
This doesn't stop them from developing it in the future, what saddens me is that there is no inkling of this type of future from any of the FD developers; i.e. the transparency is finally starting to murk over.