I would hazard a guess that they stold just the tires because it would be quicker than pulling the outside dual off using just mechanical tools. The wheels are fastened with 10 bolts on a standard truck wheel torqued to around 400 pounds for each fastener. They probably did not bring an impact wrench and an air compressor, so that would mean breaking them loose by hand.
It might have just been faster to deflate the tire and pull it off the wheel.
Or maybe they just forgot to bring a big enough socket for the fasteners.
angela