From driving them backwards in GT5 (even when they don't offer a reverse option) it seems (to me) to be a "PDI pragmatic" restriction based on their track-section stitching method and side marker placements (or possibly the window-dressings ie, signage... something I didn't really pay attention to when I was driving in reverse.) The side markers -ie, 'rumble strips' - are what caught my attention during my stint in GT5, since their placement of markers seems to have been based on a single direction of travel. For instance, when driving a given course in reverse, the ideal apices (apexes) on custom courses in GT5 often don't seem to match the marker placements, although they more often than not do when driving in the "PDI correct" direction.
I agree though, that regardless of what the basis for the restriction is, driving many of the courses, custom or not, in reverse can be a whole lotta fun. It might even be more fun if it wasn't considered the "wrong" direction and we could "legally" check laptimes, race, etc all the courses in reverse as well as the "correct" direction.