Just because an officer doesn't say have a nice day doesn't mean he's a dick. Can you imagine all the idiots this guy has to deal with on a daily basis? It's the same with police officers. They also get a bad rap because they aren't super polite or "nice" to people. They get disrespected and abused and lied to on 95% of their calls. You probably wouldn't be the most cordial person either if you had to deal with the crap they have to.
Should they be cordial? Yes. Is that what they get paid to do? No. Don't let it bug you so much.