From what I've seen, roadbuilding actually uses a (hidden) 0.25 movement to build.
So whether you have any movement remaining on the roadbuilder unit specifically, will allow you to build a road or not (note: if your roadbuilder has 3 movement, and he is in an army of 2 movement, you generally are able to build roads with the roadbuilder).
Is is definitely odd though, and I wouldn't mind seeing it fixed.
