"American Science & Engineering ... has sold U.S. and foreign government agencies more than 500 backscatter x-ray scanners mounted in vans that can be driven past neighboring vehicles to see their contents, ...."
Likewise the image on the 2nd page of the article of an SUV with the driver still inside.
Some of AS-E's trucks do both, apparently.
I agree, these vans could probably be used in occupants-still-in-vehicle mode, but there are a lot of reasons why taking the occupants out will probably still be done. In a bomb situation, you want them under your control and not able to trigger it as readily. You also get a cleaner scan with less human obstruction in the vehicle; humans are going to show up as solid objects in the scan, so hiding other stuff in the middle would be a threat.
Which is to say, negative quantities of trust.