If I hear a manager fires them at such a moment, it already gives me an idea of what kind of manager we're talking about.
If a manager sends a text message half an hour before the talk starts to not give the talk, I definitely know what kind of manager it is.
You have 2 kinds of managers: The ones that think ahead of the time, and the ones that don't think ahead of the time. It's pretty easy to distinguish the two.
Let's say Employee A comes up to Manager three months before Defcon, saying he'd like to do such and such. Manager doesn't like it, but he doesn't want to upset A at that time because Reasons. He says "sure bud, you go ahead, I'll check with the lawyers just in case and let you know if there is any problem". Then he waits, and half an hour before the talk, through an indirect medium like email or text, he goes "sorry man, I only just got a message back from the lawyers that you can't talk about that. Totally gutted! Oh well, better luck next year, uh?"
30 mins before the presentation, after they've all flown to the conference, is not incompetence; it's malice.