That notice is not a legal restriction, nor a rule to stop stores from breaking up multi-packs. What it’s there for is to alert buyers and cashiers that the barcode printed on each unit contains the product code for the multi-pack. If stores want to sell individual units, they just have to re-label them, or at least not scan the barcode.

The store in which I used to work sells individual bottles of beer, or build-your-own 6-packs. The liquor department manager just puts a slash through the barcodes with a black marker, so they won’t scan on the registers.