The Spurs do less driving and dishing that other teams do, so there are less occaisions for them to draw fouls. Horry shooting treys doesn't result in a foul more often than not. None of the Spurs look to create contact.
Iverson is the type of player that draws a lot of fouls since he drives the lane and looks for contact. When Tony Parker drives he's looking to pass or shoot, not draw a foul shot. Same thing with TD in the post, he tries to make a shot, and doesn't worry about drawing a foul.
This is a good thing for the Spurs, since Duncan is not a good free throw shooter!