The way I've always heard it stated was "The NPS doesn't believe battle reenactments are the proper way to honor fallen soldiers." which I call bs on. I think the NPS got sued during the Centennial events in the 1960's and said no more.
Either way its extremely foolish given the great number of dedicated reenactors nowadays who do things right, (contrary to the Centennial years), and how the Parks always seem to have funding problems cause if handled correctly, a reenactment could give an economic boost to an ailing Park. Not to mention get a local festival going and thus people visiting the parks and learning.
Now a lot of Parks do have Living Histories with them being invite only, and in theory a discriminating eye to make sure quality impression having reenactors come out. Arkansas Post was going to have one that wasn't necessarily open to the public this month with a skirmish off property, but not off battlefield, and it got postponed. Those type of events tend have the reenactors raising money to donate to the park, which you'd think would give some of the powers-that-be a clue...
There's a lot of NPS staff who are reenactors, a lot of whom I've known were stitch-counters, and shouldn't be tarred with the same brush as all NPS staff as those types tend to top notch great and dedicated people.