Polo Rules

At the same time it seems pretty bizarre to outlaw going for the ball above neck, but hooking a mallet above neck... Or in fact going for massive swings is fine (you're not trying to contact an airborne ball).
Agree.

We've been through this so many times. Most shots, other than tap-ins, require a follow-through or back-swing that could start or end above neck height.
Disagree. Good technique and physical conditioning are easily a match in terms of power and accuracy for the vast majority of wild swings in polo.

They manage to hit the ball pretty hard in hockey ( both ice and field) without seeming to be impeded by their high stick rules.

They hold their sticks with two hands!
This is true, but it doesn't mean one can't keep a mallet below a given height with only one hand.

Also, NHL says: "The determining factor is where the puck makes contact with the stick." You can't hit a puck out of the air above shoulder height, but you can wave your stick around without going for a ball or hitting a player.
This also is true, but...

If I wave my mallet around really fast, I don't intend to hit anyone, but it would sure dissuade anyone from entering the locus of pain.
...which is a very important point. The rules shouldn't exist only to provide redress for (what can be potentially catastrophic) moments of actual incident. They should lead to a game whereby the conditions leading to such moments are reduced.

I think that the follow through on shots is the most dangerous way that mallets get to head level, most head injuries I've seen have been either from wind-up/follow-through or from hooking mallets going above head height (something which I've got to admit happens to me quite a lot) and it seems strange that the rules wouldn't try to put a stop to this.
Agree.

This is not to say that mallets should never go above a given height, just that if they do, and in a manner that jeopardises the safety of others, the offender shouldn't be surprised if they get called on it. [/brokenrecord]

I'd like to see players get called for high stick waving goal celebrations.
You're just envious.