I can think of some reasons. None of them good.
I like how the gladiator that was on the spike strip left the cover of the car he was cowering behind to stand in the crossfire. Some of their training must involve a lobotomy.
I got this from the comments:
"STOP AND WE'LL SHOOT!!"
I may be wrong, but I think I detected some low impulse control on the part of those highly trained and nerves of steel po-leece men. Killing frenzies don't differ much between the varieties of vicious predators, be they fish or beast.