Didn't they look where they were going?? Sorry, but unless its hidden in brush, its pretty hard not to see a snake.
I have stepped over a rattle snake while hiking twice, and I was looking. My vision is better than 20/20. On one occasion, the hikers in front of me stepped right over it, and I saw it as I was passing over it. Seeing all rattlesnakes requires a higher alert than is realistically possible in my experience. In this particular case, it's hindsight. During the moment, it's not like they were scanning the area perfectly 100% of the time. The mind is lazy if one hasn't seen a snake in years or ever.
Like most people, I only see a rattlesnake AFTER I am focused directly on it. (Also, rattlesnakes don't always make noise, especially if it's a little chilly out.) Even if a rattlesnake is in the middle of a sidewalk or street, the color and pattern are still NOT highlighted in my brain. Their camouflage is the real deal.
Rattlesnake Camouflage 1Rattlesnake Camouflage 2 Those photos give me the creeps!