Easy, it's cause they're trying to sell their rifles at a very cheap price, possibly even below cost. Then once you've got the rifle, all the little add-ons are where they make their money.
Depending upon how old you are (you need to be >45 yrs to remember this) you may remember back in the 60's when Polaroid made one of their real nice Land cameras. Those suckers where you took the picture, ripped out the film, waited 60 seconds and then peeled off the cover; real slick. Well right before Christmas they were selling those cameras for about $8-12 bucks. What a great gift! Till you found out that the cost of 15 exposures was $17!!
Police Department administrators when looking for a new type of rifle to buy for their Department, may only look at the purchase price of the rifle before they make their decision (without ANY considertion for accuracy, reliability, ect...). They probably won't look at the purchase price of accessories. Police Administrators probably don't know that if you buy a dozen rifles for a SWAT team, you MAY need to buy more then one or two magazines.