It is more about permissive attitudes towards allowing these structures to come up on a communal property and not having strict or anal regulations around aesthetics.
I, as pedestrian during a windy day, would very much prefer anal regulations that would prevent a cat ladder falling on my head.
Or a cat.
No regulation in the world can prevent hat, as cats will always find something that remotely resembles a cat ladder :)
Well the Cat Ladder owner is of course liable for any damages caused. That's incentive enough to ensure people attach them firmly.
I've never heard of anybody being hit by a rogue cat ladder. They start close to the wall of the building, so are not easily dislodged by wind. Besides, even if the owner is not concerned about pedestrians, they tend to be concerned about the safety of their cat.
Flower pots, OTOH, are an entirely different thing…