"I don't know why, but people have very strong opinions about water treatment"
Part unfounded fear and part well founded caution in many areas. If you have ever had Giardia, or it's big brothers, Bacillary and Amoebic Dysentery, you will tend to error on the side of caution. Bacillary Dysentery in particular comes on pretty rapidly and will put an end to your trip in short order. Amoebic Dysentery takes a bit longer to develop but, let me tell you from personal experience, you don't want to go there. Fortunately, neither of these pathogens is a threat in US mountain areas, to my knowledge. They do need to be taken into account when travelling in certain areas overseas.