Police bikes are usually mid-range mountain bikes which can be easily obtained, and which can support a fair amount of weight, and withstand abuse. Not only does the bike have to support your weight, but add the weight of a sidearm and ammunition, two magazines, pepper spray and/or Taser, cuffs, baton, radio, mike, flash light, plus the weight of the belt, holster, and cases, then there is the weight of the safety vest worn under the uniform. And most bike cops I used to know were not of the lean variety.
The bikes themselves need front and rear lights, and rechargeable batteries, a rack with a case or bag to carry things like citation books, a notebook, pens, an extra light, and other odds-and-ends.
Though I love cycling, and worked in law enforcement, I had no desire to be on bike patrol.