It's just ignorance.  My mother is totally averse to science and would probably believe a perpetual motion machine could exist if the huckster was confident and convincing enough.  But I'd trust my mother with my life.  There's a big difference between being stupid and being trustworthy.

But it's up to you.  If you don't trust the guy, you're well within your rights to give him a neg.  I just think it's pretty harsh.