If someone complains about a dog drinking from a water source, I interpret it to mean that that hiker isn't prepared to either drink water from that source or to treat that water. That same hiker must be OK with wildlife doing a little bit of everything in the water, but will draw the line at canines drinking there, and that represents an unrealistic idea of backpacking. What about all the nasty cups, bottles, ZipLocs, hands, lips, tubes, and other various containers that are dipped into those same sources? They are all contaminated - far more than a dog's mouth. Small scratches in plastic harbor lots of harmful stuff.