Are children's pajamas expected to just burst into flames or something?

I've been noticing the children's sleep ware warnings mentioning something like "For child's safety, garment should fit snugly. This garment is not flame resistant. Loose-fitting garment is more likely to catch fire." I also see on by-the-yard polar fleece that the fabric is not intended for sleep ware since it isn't flame resistant. So, what's the deal with kids' pajamas needing to be snugly fitting or treated to be flame resistant? Is there just some expectation that these pajamas will burst into flames in the middle of the night? What's the rational behind it?