If they don't have chemicals they have to be tight-fitting. That can be a problem...one of my babies was so chubby I couldn't get him in the "snug-fitting" thermal pajamas from Target. Anyways.
Well, actually that's not quite true. Fabrics have to pass a burn test; if they don't, they have to be snug fitting. Most synthetics pass the test, most natural fibers do not. Which is a law that I think is stupid, but that's not the issue here.
Anyways, from a legal perspective, yes, it would be a bad idea to sell pajamas that do not meet regulations. But you *can* make pajamas from normal materials that do meet the requirements. Personally, I sew, and would buy, pajamas that don't meet the regulations. As a parent, I feel like my children do not require flame-retardant pajamas to be safe. However, I personally will not sell any pajamas. I think there's too much risk. But that's just me. I don't feel like taking on the Federal government.
I'm afraid I don't have a link for you, though, to the regulations. I have found them before just doing a Google search. What I've written is based on what I found looking at this stuff about 6 mo ago.