Handsomedan:

 

I think the rule of thumb being applied here by the Plod is thus: 

 

Ride that thing like an idiot and we'll bust you and give you a good talking to about the dangers of being a complete knob. 

 

Ride that nice scooter/skateboard/monowheel etc sensibly, whilst wearing appropriate safety gear and we'll nod appreciatively and may even enquire as to where you got it, if it's even slightly unusual. 

 

Seams you are largely correct, but not really what we want from a lawmaking point of view.

Having a large number of existent but rarely enforced rules makes corruption, ethnic discrimination and the likes easier for police forces. - Just pick out your target and sting them on the widely broken law.

If the law isn't appropriate enough to enforce on a regular basis, it is not appropriate enough to have at all in my opinion.