There's a pretty big jump in perceived criminality between violent crime and property crime; humans are social creatures, after all. Far more people are willing to steal a car that's parked on the street than to hijack one, far more people are willing to commit burglary than home invasion, etc.

Even for the people willing to use violence, having a human involved opens up unpredictablity and risk. You can't know what's going on in the head of a random trucker, whether they're armed, whether they fancy themselves a cowboy, whatever. It's a high-risk, moderate reward scenario.

In contrast, if you have a system for consistently hitting autonomous trucks, it's a (relatively) low risk, moderate reward crime.