It's a requirement by the licensor of the images that the fabric manufacturer print this on their fabrics. It's a limitation on who the manufacturer can sell the fabrics to. The manufacturer (and distributors) are not allowed to sell the licensed products into manufacturing situations.
Once it gets to the retail level, and thus to the crafter, the statements on the selvedge are no longer applicable.
The thing is, it's showing up on fabrics that are NOT licensed images at all (like polka dots and florals), just plain ol' ordinary copyrighted fabric designs. Most of us are okay with limits, perceived or actual, on licensed designs, but when someone says you can't sell a quilt made with a red and white polka dot that has 'not for commercial use' on the selvage (and no mention of licensed anything anywhere), that's just plain absurd so why print it at all.