At what point in history does it become acceptable to dress up for military renactments? What about wearing a German pickelhaube from the Great War? 

3-Ducks; WW1 and WW11 festivals remember our nation's part in the conflicts, so any uniform of the adversary would be inappropriate unless there's a re-enactment.

Naturally, Nazi uniform is a no-no unless you're making a film or documentary.

WTF those dolts were thinking, one can only imagine. Indeed, they weren't thinking at all. They were definitely Daily Heil /Sun/Express 'readers'.