Interesting how when the news media baselessly questions the motives of whistleblowers it's a "smear campaign." When someone on HN does it to someone who's vaguely pro-NSA it's the top comment.
I guarantee you that this is not something the Feds themselves believe.
What makes you think it is true?
The charity is involved with educating the public about the recent heroin epidemic that's been hitting the country pretty badly. Now lets suppose that NIDA or the DEA was involved with a scandal having to do with letting dealers off the hook or fudging some study. And lets suppose the prevention and treatment community, in response to this, announced that no Feds are allowed to attend our annual event (which is real) where we always invite speakers who work in the area of prevention, treatment, and law enforcement. That would not be the right thing to do. Those Feds who normally come to our events have a genuine concern for the cause and offer valuable insights. By not inviting them the organization may make a strong political statement but its really not doing anything except allowing a small group of people to pat themselves on the back and say "yeah, we showed them". Meanwhile all of our attendees and guests are missing out on valuable insights they may have otherwise gotten had the organization not taken this route.
There are betters ways of making political statements. DEF CON themselves could have tried to do something to address the NSA leaks issue at the event without barring a group of people who most likely don't even consider themselves as Feds when they're in the context of a security convention.
Or, could also be because governments normally pay lots and lots of people for holding opinion, while normally there is nobody paying for the opposite side.