I'd like to give my opinion on why it is that those portraying certain nationalities act the way they do.
My main impression is The British Tommy... mainly because I am English. Naturally, I can portray the period caractor to perfection, and others of my unit take their lead from me. Its an act, and all a part of a full immersion into the national caractor. I feel that for most people, to actually act out the mannerisms of the national impression they have chosen... is all a part of the enjoyment of doing that particular impression. Its all about 'getting it right'. Getting the feel for the part. Think of it this way... the individual is rehearsing for an audition, in order to get the part of a certain caractor in a play. To get everything right... clothing, mannerisms, speach... all adds to the chances of getting the part.
To my way of thinking, I expect those who portray French.. to act the part, Germans to act likewise and so on. It all adds to the realism of the event... the pariod authenticty... the accuracy. Just getting the uniform and equipment details right.. is only a part of reenacting.
Thats just my view!