Normally the dealers receive crated bikes dry (no fluids), and they then proceed to fill it with oil and other fluids. I'm only assuming here, but I would think their contracts with the bike manufacturers stipulate that the manufacturer's branded oil be used, and again I'm only assuming, but I would think that many dealers will use whatever oil is the least expensive for them and therefore best for profit margins.
Bel-Ray is using "mineral" to mean "conventional" oil, i.e. not synthetic.
Using this terminology is confusing because "mineral oil" normally refers to the food-grade, odorless, colorless kind, which is commonly used as a laxative, as a make up remover, and as a cutting board conditioner.