In case he doesn't post it here... A post by bmarnz:
If it is indeed the case that you still receive gold from foreign franchises than I consider that a completely valid "in-game" reason to be able to build them when running SP.
Bh
Now that you've changed it, I'll take the other side. If your running SP, doesn't the founding city still receive gold from foreign franchises. It'd be like China today, a communist country exporting trade goods to the world.
If it is indeed the case that you still receive gold from foreign franchises than I consider that a completely valid "in-game" reason to be able to build them when running SP.
Bh