The best that I can tell, defense is intended to reflect footwork and shotblocking the likelihood of blocking a shot. Any player with poor footwork is going to be reaching with their hands; thus committing more fouls. I think that the sim fails to take into account that, in any defense, a guard, taking bad defensive positions, may simply get good at not committing fouls, but not taking better position (the old "olé!" defense). That ought to yield more fouls committed by the forwards instead of the guards.
Anyway, I have no idea whether shotblocking is simulated to affect shots that are not counted as blocks or to increase the likelihood of a foul committed on an attempted block. I think that the typical view is that in M2M (and to a lesser extent fcp) the defensive rating is accentuated, while in zone you may be able to have a center with a very poor defensive rating, but would want the high shotblocking. That would make sense as, in college, the center never has to exit the defensive paint.
There have also been some posts that suggest in zone defense the defensive ratings are averaged. If so, you could afford to have a C with poor D as long as the total average D on the court was still good. I am not certain whether that is true either.