Because chess has a throughout specification (at least now, not sure if it was always that way). A knight is only a night and cannot fall under any other category. Can you think of a rule in chess that is not well defined?
Chess rules are also different from real life, but people don't have any issues following them.
The question whether it was “well defined” doesn't come into play here, I think: the rules in the game tell you to ignore any overrides (such as that certain vehicles may be allowed in an emergency situation). And GP decided to ignore that part.
I think interesting discussion can be had even when respecting this rule. For example, I decided that a skate board is not a vehicle but a bike is.
Lets take chess and the knight, and 2 - 1 movement rules. If there are pieces in front of the knight you pick up the knight off the board in 3 dimensional space, right?! The rules of chess only define the final end point of the piece and not the physical motion in real space to reach that location.
With this said, if you're in a professional setting, and you pick up the king and fly it around like an airplane, everyone is going to get really tired of your shit.
Naturally, this rule is not interpreted literally.
Edit: I think in tournament chess there is some ill defined rules around play behavior/distractions.
Where the contention occurs is on human behaviors. Where the rules for chess are pretty simple (and yes, this does lead to difficult to compute and complex behavior), the motivations that humans follow both individually and as social structures are far more complex and contradictory.