I'm not looking for "because the rule says so". We're told that the best way to learn the rules is to understand why they are the rules and what they're trying to accomplish. I now know, for instance, that a low block towards the opposite sideline is frequently very difficult for a defender to brace for, which can lead to serious injury (and is therefore illegal); while a low block towards the near sideline usually has a completely different angle and is much more safe to be on the recieving end of (and remains legal).
I am having difficulty working out why high-stepping is specifically prohibited and raising the arm is not; I can see no practical difference between the two acts.
Have you ever heard the saying that armies always prepare to fight the LAST war they fought?
Rules are the same.
One is illegal and the other not simply because when the rule was written, players high stepping was a common way that they would draw attention to themselves. The rules intent is to stop this kind of grandstanding and attempts to focus attention on the individual rather than the team.
So we get rules that try to do two things:
1. Stop particular instances of known behavior that is definitely not ok (high stepping, spiking, pointing, throat slash) while
2. Being general enough to provide officials with enough latitude to penalize similar actions that are not identical to the listed actions, but close enough that they are effectively the same. This is to make sure that if you ban high stepping, someone does not subtly alter the behavior so it does not meet the specific definition of the rule, while still violating the obvious spirit of the rule.
This is why we get language like "acts include, but are not limited to..." in the rules.
The problem is that we will always have competing issues with rules like this. On the one hand, you want the rule to be general enough that officials can use their judgement to penalize appropriately without the need to explicitly list every single possible way someone can be unsporting. On the other hand, the moment you make a rule generalized and count on the officials judgement, it is 100% inevitable that somewhere, sometime, some official will apply that judgement poorly, and penalize something that the rule likely did NOT intend.
There is no solution to this problem, you can only minimize it.