Who would ever get a bumper sticker like that for the sake of irony? Who would ever do that, not having read somebody's email?
Having thought about it, I can answer my own question. People whose job is much more invasive than reading people's email. Like, if my family and neighbours knew I had some secret spy job, but they didn't know that my job was to spy on you with a hidden camera in your bedroom, then I might slap that bumper sticker on there just to give people some bait to latch onto. I suppose you could call that irony.