Civil Servants / Public Domain

Over the years I have recorded many different conferences ranging in focus from the Arts and Public Policy to Technology and Medicine. Often there are civil servants invited to speak at these events, yet all of the civil servants decline to have the speeches filmed, where as pretty much everybody else seem only glad to participate. Does that not seem wrong? Surely as civil servants what they say is in the public domain simply due to the fact that it is that very public that (over) pays their salary and expenses incurred to attend said event?
What are they afraid of?