I think that those racks allow SWAT/paramilitary officers to stand while the trucks drive in to deploy those officers quickly into situations requiring rapid response but not armored transport. I'm a little surprised by the differences in the cages (2nd truck's is higher above the light bar than third etc) and by the seeming 2wd and lowered ground clearance though.
I would have expected personnel carriers to have upgraded bumpers and 4wd...