I would not say cops are obsessed with donuts, but you eat what you can get.
If you work in a Precinct that has good places to eat you eat good. If you work in some bad areas where there sometimes is no good places to eat you eat whatever is available. I worked in areas where fine dining would be White Castle but I also worked in places where I was able to eat any type of food I wanted because there were so many choices.
Anyway, it comes down to personal choice. I worked with people who ate junk food all the time and others that ate healthy.