Cops do it mostly because they are the only ones professional enough to do it.
Yjeu gove other reasons, like the weight of the gear on their bat belt is too much and they are constantly needing to pull it up, or that because of where they have the holster "where else am I going to put my hands?" and the like.
In other words, it's all a load of organic manure.
One way of dealing with this behavior is to videotape them and present the video to their bosses (city council or whatever the county equivalent is called) along with a public plea for action to stop the behavior. Be sure to notify the press ahead of time and to have CDs/DVDs to hand out to the elected officials, the press, and the general public.