Do they have to contort themselves into special positions that an officer on a SWAT call out never would? If not by your logic there is no reason for military personnel to wear the ninja designs either.
Several years back I bought myself a set of surplus BDU's for working on a short line railroad (all the live long day!). If I was a wanna be I could have simply reenlisted. The reason I got them was I was doing track work and needed durable clothing that wouldn't bind when I had to squat and bend. They also had plenty of pockets for things like spikes, water bottles, first aid kit and bug spray. They were simply cheap, durable and comfortable. The reinforced knees and seat held up well sitting and crawling on the track bed. A pair of Dockers wouldn't last five minutes.
So believe it or not, sometimes folks don't really care what they look like, they just want what is the best tool for the job at hand.