The difficulty is this. Working as a police officer today includes doing many things that a person with vaguely libertarian views regards as outright immoral. No-knock searches are just the tip of the disgusting iceberg. On the other hand, police are also first responders. They are people who run towards the gunshots. (At least in theory).
What these people are advocating is by no means unamerican – it is also the same thing pointed out by various conservative, libertarian and anarcho-capitalist authors – that our political situation is in a stage where armed rebellion might soon become justifiable, and it would then be fine to shoot at certain law enforcement agents. A lot of authors toy with this idea, deliberately coming as close as they can to saying shoot the cops now without saying it on purpose.
I don't personally have a respect for law enforcement (which is not the same as saying I have no respect for the concept of law). This does not mean I think we should commence methodically shooting them. I just personally don't respect them, would prefer not to hang out with them, have them as my dinner guests, etc. This is my right.
I also believe – as anybody in his right mind does – that if you are a member of a violent attack otherwise unprovoked – you have the moral right to resist violently. More to the point, people will resist violently. If you take it as your career to burst into people's homes unannounced, throw flashbangs and point guns at people, there is a certain risk of being shot. Such is life. And I believe that the people who might shoot you should be acquitted – whether or not you are a police officer, a crook, or Buddha himself.
What these OWS people doing is stupid as misguided. This specific stupid, as I mentioned, is not a particularly left-wing stupid – it has been attempted before by members of the right as well (surely you've seen posts of this nature on pro-gun forums!) - is comprised of people who are all (REASONABLY!) afraid to actually start a civil war first, and are all standing around trying to poke at their friends and neighbors trying to persuade THEM to go first (rather foolishly).
There's always the chance that if you decide to start a civil war you'll go down in history as that guy who went crazy one day, shot two EPA administrator and got put down by a SWAT team, and either discredited his cause or got forgotten by the news cycle in five minutes. Nobody wants to be that guy. Everybody wants to be Camille Desmoulins.
There are of course great moral reasons why we shouldn't start a civil war – because there are peaceful alternatives, and because it wouldn't work and would only damage our cause when it fails (applying to virtually any cause, really).
These people are stupid, but their stupid is not unique to them in this context.