Epic WTF

Nope. There's nothing inherently dangerous about cycling past a car, assuming that the cyclist can do it without falling off. The danger is not from some naturally occuring hazard, like a volcano, that can't be prevented, it comes from the person in the car not paying sufficient attention when opening the door. So although we could focus entirely on the perpetrator of the offense (vs. Section 239?) we choose to tell the potential victim how to stay out of harm's way.

Well, not exactly. There's nothing inherently dangerous about cycling past a car, but there is an inherent danger in cycling past a car very closely - you keep your distance from parked cars partly to avoid hazards like doors opening, partly so that you can see more of the road, and partly so that you have time and space to react to unexpected events like dogs or children or grown-ass adults who have no idea how roads work suddenly appearing from between parked cars.

You're also expected to keep your distance in a vehicle, and it composes part of good road positioning and should be regarded as normal. Poor road positioning is specifically dangerous because it reduces your ability to react to sudden hazards (including but not limited to doors opening in your path), and it's discouraged in favour of good road positioning, which is functionally identical to bad positioning - as you still get to go where you were already going, at the same speed - but removes a component of danger.

That's not to say that people who open a door into somebody's path shouldn't be prosecuted, as it's still an offence.