I asked a CHP last year about the helmets and gear. They aren't allowed to wear full-face helmets and heavy gear because it intimidates the public, and cops are very big on public image. The CHP I spoke with was in favor for wearing flip-ups, but they were not approved for wearing (yet).same here!
I have no idea why they do so. I could understand the 3/4's helmet (for speaking to the public), but they could use flip ups?
I guess that they're just heavily trained to ride the bike.