I think that forums heavy with computer desk jockeys like HN tend to throw this idea around too casually.
If outsiders kept callously telling us "you signed up for it knowing that you would work 80 hours a week" or "you signed up for it knowing that your stock options would probably be worthless", we'd be up in arms and demanding that no one should be treated that way.
Yet we expect officers to put themselves into situations where they'll be hurt or injured unnecessarily. Take, for example, the Eric Garner case: "They didn't need 5 officers to tackle that guy who was resisting arrest". Really? That was a big dude. If he had decided to start punching; officers could have suffered knocked-out teeth, irreparably damaged eye sockets, and broken bones. Can you imagine going to work every day where the public expectation is that the risk of your knocked-out teeth when confronting someone resisting arrest is perfectly acceptable?
There are very few jobs I would never do, but being a police officer is one of them. The public is just way too callous in thinking that it's okay for you to be injured or killed because "you signed up for it".
[grammar fix edit]