I think a few different issues should be considered here:
1.) Playing Airsoft in a Second World War bunker.
This is fine in my opinion, sometimes.
But to me, there is a difference between an old reserve airfield bunker in Lancashire, and the business end of Fortress Europa. It's all about context...
A cops vs. robbers Airsoft game at The Mall, Reading - no problem, yeah? But how about a cops vs. robbers Airsoft game at the Westgate Shopping Mall, Nairobi? There's a difference...
2.) People dressing as soldiers to play airsoft.
Are people dressing as soldiers, or characters? If it's the former (and of course some do), I think that in itself is often distasteful. If they are dressing as characters in a game however, it's different (to me at least).
I guess it all comes down to the various shades of mil-sim vs. skirmish on the sliding scale of acceptability.
Mil-sim may entail NATO vs. terrorist - a pretty superficial concept, not much more advanced than a child's goodies vs. baddies approach - its just a vehicle for one group of players to play the game against another group of players perhaps. But when people start dressing up to play 40 Commando vs. Taliban Musa Qala forces or the like, I think it crosses a line...
3.) The combination of these.
I can't get my head around the scale of trivualisation that the 're-enactor' core of Airsoft descending on the Normandy bunkers to shoot plastic at one another would be...
I don't mean to get preachy- but I'm just trying to say that staying on the palatable side of that sliding scale of acceptability is often down to the context, and recognising where distinctions should be made.