Why must infants must be placed on there backs?

So they don't suffocate.

Explanation:

If an infant was placed on its side then it could roll over onto its stomach and possibly suffocate itself. This is why many parents do not but stuffed animals with smaller, younger infants. They do not have the required muscle to save themselves from something so simple.

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.