To answer your first question - I'd posit that (especially for North American readers) the big driver is a lack of credibility in the FDA, and the 'health industry' in general. Look at the evolution of the Food Pyramid (as probably the most well known example) over the years, but also the lack of controls on where HFCS ends up (ie. just about everywhere), alternating between demonising and lionising various food stuffs from one decade to the next.

Any kind of scepticism on this front is probably (pardon the pun) a healthy thing.