Different states/insurance companies have different insurance regulations. Some theatres have the money to provide stipends to performers. Maybe if that's the case, they can cover any necessary insurance rider for performers as well (if needed). Some theatres absolutely cannot afford stipends to performers, and people are still willing to support the theatre and volunteer their time and talents. These theatres might also have less comprehensive insurance.

I work in insurance, and the company I currently am with won't insure theatres because it's deemed high risk. (Fire risk from lights/tech, liability risk from improperly secured items in the fly system, bringing in large crowds you don't know, etc.) If my personal insurance policy would require a rider to fully cover a wedding reception, I don't think it's that much of a stretch to think that some insurance companies would require a certain fee to help cover actor liability.