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Because cyclists are the ones who would (if they didn't know any better) be cycling in the door zone. There's nothing insane about it.
The poster is educating cyclists that it's dangerous to ride in the door zone. Now, cyclists may currently do so because many cycle lanes put them in the door zone, or maybe because they just don't know any better/hadn't appreciated the danger. I don't personally know any cyclists who've been bullied into the door zone, but fair enough, I'm sure they exist.
Hopefully this poster will also educate drivers who don't why cyclists don't cycle right next to parked cars.
Also, however well educated drivers might become about looking over their shoulder and in mirrors before opening their door, I still wouldn't ride in the door zone. It only takes one person failing to notice you to door you.
Why is it aimed at cyclists at all? It's insane. The two issues are people opening their doors into traffic and the attitudes of many drivers towards cyclists in a position away from the gutters. And yet this advise suggests cyclists are playing some stupid fucking game.