Your confusing freedom of speech with the practical rights of citizens in a crisis to clearly know who is and isn’t a law enforcement officer.
Your freedom of speech does not include yelling “Fire” in a crowded theatre. Common sense has to have a place here.
Do you not understand irony, that you would take such a bumper sticker literally?
Sorry, but police do not have a license to infringe upon a person's constitutional rights just because the cops and 47% of the population are too stupid to get the joke. This reminds me of the premier of the old Sarah Silverman Show on Comedy Central, when she is pulled over by a cop who asks, "Do you know why I pulled you over?" She responds, "Because you graduated last in your high school class? Its the only funny thing that Silverman ever said on an otherwise forgettable show.