The part where the "all", by legal precedent, is understood to apply narrowly to the signatory parties --- not to those not party to such agreements.
Think about it for a while. Try putting it in a different context, and think on how that may apply as a neutral principle of law.
Are you responsible for what some other person does? Can you, yourself be made to be contractually responsible for the acts of others---which you may have no control of? How would THAT work??? Would it be right & proper?
The promoter of the event should have made sure that all promotional materials were approved as per the contract they signed.