I wouldn't trust even a bubbling spring in a popular camping area. I had a friend get giardia (confirmed by laboratory tests) from the two nights he used a spring bubbling out of the ground. Those were the only times he didn't filter. It took him almost 6 months to get over it. The spring was in a popular camping and hunting area (Buck Creek Pass in Washington's Glacier Peak Wilderness). I can only assume that the ground water in the area was polluted from improper disposal of bodily wastes.
It takes about 10 days' incubation for giardiasis, so anything that hits sooner is either due to other stuff in the water (coliform bacteria, for instance) or, as Lori says, poor sanitation.
Since I started backpacking as a little kid in the early 1940's and never treated my water until the mid-1980's, I wouldn't be at all surprised if I'm immune to giardia and maybe other stuff. After seeing how miserable my friend was, though, I have no desire to test this theory!
EDIT--another ULA Amigo Pro fan here! I do take Micropur for backup and for treating really questionable water (especially if stagnant and frequented by livestock, deer or elk). I don't let my dog drink stagnant water in such circumstances either.