There is alot of room for bias here. Is the fake foot a realistic proxy of an orgional? Can the snake recognize the difference? You don't fistfight a book or other object falling onto you. Would a snake bite a rock that shifted onto it when it bumps into it?

How much pressure is the fake foot applying? A person accidentally stepping on a snake would apply their whole weight because they didn't know it was there. Likely hurting the snake and warrenting a more significant and immediate response. I doubt the fake foot was applying the same force because of ethical reasons.

Edit: Phrasing

Snakes only produce a small amount of venom and it takes days to replenish reserves. They have fine muscle control over how much venom they release and scientists have measured them releasing amounts of venom relative to the size of their target.

Not sure how aware they are of doing it but I wouldn't be surprised with all of a viper's sensory organs that they knew venom would go to waste on a prosthetic

I think the biggest factor is pressure. A person would put their full weight down because they don't know the snake is there. This would probably hurt the snake and warrent a panic/significant response. I doubt the fake foot was applied with the same pressure because ethical reasons.