Off of the top of my head, being, perhaps, criminally liable for not reporting in an instance when the volunteer wasn't sure might discourage volunteering. If I were contemplating helping out at the local community center, and I knew that I might be charged with a crime if a kid was acting strange or withdrawn, I would certainly be dissuaded from volunteering. Because while I might be saving a child from abuse, I might also be needlessly meddling and required to do so by law.

Now, I don't know what the repercussions are for not reporting as a volunteer - I'm just speculating.

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"Government big enough to supply everything you need is big enough to take everything you have... The course of history shows that as a government grows, liberty decreases."

-Thomas Jefferson