The volunteer work is non critical. Thats how they get away with it. (thought the cleaning up a patients room is highly questionable activity)
Most volunteers I've come across either man stations where they give people directions, offer then snacks, do things like translate for families who don't speak english well (for non critical things, like in the cafetaria or the parking garage, not for talking with doctors about medical problems). The most I've ever seen is they can sometimes wheel patients down from their rooms only when they are being discharged home and even then they need training before they are allowed.
Like I said, non critical jobs that if all the volunteers called in sick would go on without anyone noticing they all called in sick. (there are already professional direction givers, already professional translaters, already professional wheelers of patients and more then enough to do the job on any given day)
To be honest, most of these volunteers (assuming they are doing it because they are interested in medicine or hospital work of some sort) would be better of just finding a doctor who works in the hospital and asking if they can shadow him regularly. Then they'd be in operating rooms and cathether suites seeing (but not touching) cool stuff rather than doing work that no one really cares about or would miss if they were utterly useless at it.