As you have said it comes down to the officers discretion.

Unfortunately there are no listed guidelines that states "Flashing highbeams at another car from infront or behind is outright illegal".

What you may classify as road rage may not be what is actual road rage.

I am not saying you are entirely incorrect, I am saying that your claim that it is outright road rage is outrageous!

It comes down to the officer investigating and as there are no direct guidelines it is very much so a grey area.