Everyone seems to think that the Dark Tangent is making a political statement uninviting the feds to DEF CON. Maybe, but there's nothing political in his message. Whatever politics you read into it are those you brought with you.
People who run things, from corporate CEOs to con organizers, learn to keep themselves above the fray. They spend a lot of effort heading off conflict before it has a chance to start. They don't take sides. Those who are wedded to their side are sometimes unable to recognize this impartiality.
A highly visible fed presence is likely to trigger conflict with people upset over Snowden-gate. From shouting matches, to physical violence, to "hack the fed", something bad might occur. Or, simply attendees will choose to stay away. Any reasonable conference organizer, be they pro-fed or anti-fed, would want to reduce the likelihood of this conflict.
The easiest way to do this is by reducing the number of feds at DEF CON, by asking them not to come. This is horribly unfair to them, of course, since they aren't the ones who would be starting these fights. But here's the thing: it's not a fed convention but a hacker party. The feds don't have a right to be there -- the hackers do. If bad behaving hackers are going to stir up trouble with innocent feds, it's still the feds who have to go.