When I was in Iran at the end of last year, I've also planned to go to Kerman and Bam area with my car.
All my Iranian friends, that I told them the plan, said, that I should not go there because safety reasons. When I asked more accurately, what are this safety reasons, I could not get an answer, only our best iranian friends told us, that there are some tribal gangs wandering around Kerman, kidnapping tourist.
Other source said, when they came from Pakistan last year, they had military escort from borderline to Kerman.
From third soure - Slovenian overlander, who was in Iran in 2010, I heard, that they were driving south from Kerman in the desert, and they came to military camp, where some young Iranian solders, probably high on pot, were making some problems and acting dangerously with the rifles, so they turned around quickly and left the area.
What I want to say, I don't have any accurate and fresh information, but I'm not surprised, that Persian tours don't want to take any risk, especially with US citizen.
Greets