Q: Why do famous people with access to valuable secrets get paid silly money for giving 'lectures'?
A: The money is a bribe - the public payment for private information or covert influence.
Bribes are of course illegal; while paying somebody silly money for a 'lecture' is not. But the money goes from one pocket to another, just the same.
Simple, really, if you understand what is going-on, and join the dots.