Insurance companies dictate the workplace these days.
I would find that proving I wasn't a drug addict just to volunteer in a library would send me the other way. It would be better that institutions taking volunteers, would have those folks sign agreements releasing them of responsibility for injuries on the job.

I just finished a two month stint volunteering for a new cultural center here in town and trouble shooting for the core group through all kinds of family crisises and bureaucratic shuffles. I would no sooner fix one hole in the dyke and they had another one for me until I was buried with responsibility. Yesterday was my last.

Looking forward to a new start.
I'll bet there's a piss test in my future though.