Why do pajamas have to be snug-fitting, but not other clothes?

Pajamas are supposed to be either flame resistant or snug-fitting so kids don't get burned as easily--but, the majority of burns happen when kids are playing with fire. Which, they obviously aren't when they're sleeping! Why if we're supposed to be so worried about pajamas being snug-fitting don't we have to worry about this the rest of the time? I'm just not understanding the fed's reasoning here. Under 9 month olds' clothing isn't subject to this requirement because they're not mobile (info source here: a href="http://www.toxicsinfo.org/kids/toys/FireRetardantSleepwear.htm)," rel="nofollow"http://www.toxicsinfo.org/kids/toys/Fire…/a so this isn't about house fires, but about kids exposing themselves to fire. Can anyone explain? Or find the stats that show that kids are significantly more likely to play with fire between bedtime and morning? Thanks! (Just trying to decide if this one is worth worrying about--or ignoring as a silly gov't idea).