Is it illegal to request a mandatory donation of money for entry to an event?

No, event operators don't have to tell donors where their entrance fee is going. If it's a publicly traded corporation or a non-profit, they've got state and federal disclosure requirements, and attendees are free to research them. No, they don't have allow entrance to people who don't pay the entrance fee. Non-profits call these fees "donations" because the part of the fee for which no value is received is tax deductible.