Just because you are a "unpaid volunteer" for an organization does not mean the organization is free from all civil liability. Remember, Jehovah's Witnesses active in the ministry are dubbed "Publishers" and document their work on a monthly basis. As such the liability goes both ways.
In my line of work we get a lot of citizens who volunteer for us. We screen applicants for volunteer positions because our agency is liable for what they do when working for us. Likewise, if one of our volunteers is hurt while working for us they are covered by Worker's Compensation.
I see such lawsuits being successful. This may very well be the final nail in the coffin on the Watch Tower Society. It creates a near paradoxial legal dilemma for them. A person cannot be one of Jehovah's Witness without actively preaching and peddling the publications of the Watch Tower, Bible & Tract Society. Yet, anyone who is injured while doing so is an agent of the WTB&TS and therefor must be compensated for medical expenses, lost wages, etc.