I think he went to Black Hat because unlike Defcon, Black Hat is run by a large multinational media company with a PR department trained and motivated to secure keynote speakers who will attract the most press attention to the conference.

Alexander actually spoke at Defcon last year and garnered quite a bit of attention regardless, so I don't quite think this is the case.

Exactly! Actually, Alexander was asked the Question of NSA datacollection after his talk, basically it went down like this:

Q: "[..]Does the NSA really keep a file on anyone? [...]"

A: "[..] Frist, no, we don't [...]"

Absolutely astonishing!

listen/watch here: http://youtu.be/tz0ejKersnM?t=33m2s

What is astonishing by that? They can easily be splicing fiber cables taking a copy of most Internet traffic and recording either the actual call or just the from/to/length info and his answer is totally correct. It is actually a really terrible question.

The feds were not banned from Defcon.

What? Defcon basically said "Hey feds, don't show up here". They may have said someone who works for the feds can show up, but they asked that they not represent their employeers, and to only represent themselves.

Not technically the same as a blanket ban, but they were pretty clear that they didn't want anyone attending while representing federal agency...

That's not a ban. Nobody is turned away, even feds in uniform.