Monkeyleg, if you think about it...
The Springfield M1A rifle is for all practical purposes a parts gun. Until such time as Springfield, Inc. starts making all of their M1A parts, they will continue to use USGI surplus M14 parts for assembly. Take a look at the average Springfield, Inc. M1A, and you'll be surprised to see the variety of parts that make up the gun. TRW, HRA, WRA, you name it, the parts came from military contractors who built and assembled M14's. You'll definitely get a Springfield, Inc. receiver, and perhaps a bolt, but the remainder of the gun will probably be military surplus, and there will be no "naughty bits" like the connecting link and selector lever for full-auto.
The USGI fiberglass stocks that have the selector lever cutout are the same ones that are found on the Springfield, Inc. M1A. Sometimes they take the time to fill them in before painting, sometimes they don't. But they do use the surplus pre-ban stocks on post-ban guns, no legal problem there. On their wooden stocks, found on the M1A National Match and M1A Super Match, there's often no selector cutout, because they're new production wood.
You can find wooden stocks with the selector cutout. Fred's M14 Stocks sells them, just check out the Shotgun News.
On my Armscorp/H&R M14NM below, the stock did have the selector lever cutout. It was filled in and smoothed over prior to the paint job. One less place for dirt and crud to enter the receiver area since the selector lever was never gonna be installed.
Hope this helps explain the cutout on a post-ban M1A.