Why shouldn't they be there ?
Napoleonic Re-Enactment in Europs is not very big (only a few thousand re-enactors over the whole of all Europe). Actually, we already consider an event with a few hundred partcipants as "big". If you want to have something really "big big", you cannot chose pickily whom to invite.
And anyway, the "re-enactment" of the battle of Waterloo, with "battalions" of less than 200 men, tiny "units" of less than 20 men on horseback, ugly plywood dummys which were euphemistically called "Hougoumont" and "La Haye Sainte" (not to talk of the "Plancenoit" mockery), had as much to do with the real battle as the replays I used to do as a boy with my Airfix toys.
But if you think of re-enactments as a kind of Napoleonic theme parks, with enthousiasts from allover Europe meeting, gathering some insights in Napoleonic soldiers' live and drill and some of its problems, and spending their time with friends (many of them also representing other nations) who share the same interests, you will find the Austrians, Danish, Saxons, Consular French etc. in the right place ;-)