I hope there isnt a lot of fertilizer missing somewhere..
I don't think gas would work very well with fertilizer. They could make good molotov cocktails but gas evaporates too fast. The trucks themselves would make great bombs though.
Probably stole it because diesel fuel is valuable (expensive) now.
ANFO is only ~7% fuel oil -- and that is primarily to keep the ammonium nitrate from absorbing water. Those two trucks hold enough diesel to convert an entire shipload of ammonium nitrate to ANFO. And explosive-quality ammonium nitrate in those quantities is tightly controlled.
There may be some happy truckers somewhere, but I'm not concerned about that fuel being turned into ANFO...