I agree that MB had every right to ask them to sign whatever agreement he wanted. They had every right to decline to sign it.
I still find it difficult to imagine it was a straightforward renewal of an already signed liability waiver. It seems more likely that it contained some changes to the terms of their previous agreement.
Once they declined to sign it, MB had every right to end their existing client relationship.
When they declined to sign it, instead of accepting their decision and ending the interaction by walking away, MB decided to call 911 and prolong the interaction. We don’t know if he ordered them to wait in the parking lot until the police arrived. We do know that he ordered RC not to leave.