The Britax thing drives me nuts (and other CPSTs as well!)
People often spend mucho dinero (or slightly discounted dinero
) on a Britax, only to show up a seat check and have me say, "Ooops, sorry, but this doesn't work for your baby. You're going to have to return it for a different seat or buy an infant carrier for the first few months." Talk about a crushing blow to a pregnant woman!
As for the legality issue, any state that has a proper use clause (something along the lines of parents being required to follow manufacturer's recommendations when using a seat) in essence makes using a Britax convertible from birth illegal, because Britax requirements (as well as the requirements of all other child restraints) state that the child's shoulders must be at or above the lowest harness slot.
My children are average to large and didn't get there until 6+ months! Fortunately there are a LOT of great child restraints on the market that work from birth, will get kids well beyond 4 and 40, AND cost less than Britax products to boot.