Right now, impressionable youth from 3rd world countries are cheaper than robots. There won't be much worry about this for a while. A rust-bucket Honda and some dumb kid are going to be a lot cheaper than the latest Google-Tesla joint venture product.
We have plenty of time to think about it before Is-lame-oh terrorists are using them.
Except that one limiting factor in the jihad is the ability to get the starry eye idealist soon-to-be-martyr over on this side of the pond. Blowing oneself to tiny bits appears to be a hard sell to westernized folk. The concern here would be that an autonomous vehicle could alleviate that problem.
Of course, it's not a perfect solution. You have to purchase or steal the thing which now are in rather short supply. An autonomous vehicle is going to be fairly tightly regulated once let out into the wild - one of the basic tenets is that it communicates with other vehicles and presumably some sort of central command. Driving into a crowd or a chemical factory might be frowned upon.
Much easier to just blow up a chemical factory somewhere in New Jersey. Or start a forest fire near LA.
But it's the FBI's task to get all paranoid and look at all potential possibilities to ensure our ultimate freedom. USA! USA!