You don't know? Guess it's none of your concern...
I don't know either, just seemed pretty weird. Lectures are usually for a select audience, like being enrolled in a specific class, or work in a certain field. Doesn't give a where or when, but charging admission, I'd guess seating is limitted. Perhaps it's one of those 'Get rich, in your bathrobe' lectures, would explain the need for free admittance....