This depends on what sort of deception you are talking about. The first answer implies that all deception is improper and that police officers must hold themselves to a higher standard. But I do not agree with this at least in certain circumstances.
Basically, I see no reason why police should have to tell the truth to people when they are questioning them. If a police officer stops you for speeding (and you were speeding) and they say "I clocked you on the radar -- do you know how fast you were going" what difference does it make if they did not actually clock you? They are lying to get you to admit to something you actually did. I see no problem with that or with them saying (in a more serious case) we have people who say they saw you -- why don't you just admit you broke in to that store."
I see no reason that police should have to tell the whole truth in all circumstances. I think that the public has no problem with police telling these sorts of "white lies" in the course of investigations. I think that the public generally sees these as acceptable sorts of deceptions that do no harm and are done in order to achieve a proper and desireable purpose.
Deception, in any form, is typically considered lying by general standards. When I think of the profession of police officer, I want to cling to the idea that these are among America's best and most noble, doing what they can to serve the people with integrity. Of course this is not always the reality. I cannot speak from experience as a police officer, but my experience as a human is that once even a small seed of deception is sown, it tends to grow. I think anyone involved in a lie (however small it may seem) has compromised his or her ethical decision making.
Because we are taught as small children not to talk to strangers, but that police officers are our friends, it goes against what is ingrained in us when a police officer is corrupt. Certainly deceptive behavior negatively affects how the public views the police force.