Its nothing but tactic calculated to create uncertainty and fear in the consuming public to the point that many consumers feel they are at risk when using anything but OEM-branded lubricants.
GM only recommends what makes them money. They want a monopoly on what you buy for your vehicles and their owners manuals and propaganda are designed to affect that end. They cannot force you to buy something branded by them (GM) that helps them make money. It is illegal for a vehicle manufacturer to tie the warranty of its product to the use of the manufacturers own lubricant.
I think you may be taking the Magnuson–Moss Warranty Act and adding some of your interpretation to it. Or you are talking about some of the anti-monopoly/anti-trust laws. Both of those would only be applicable in the states, and I am not certain the anti-monopoly laws are entirely federal.
GM's Dexos specs are their way around these laws. Their wanting oil companies to pay them a royalty just to be able to advertise that it passed GM's testing as opposed to just merely meeting their specs, is real close to violating anti-trust laws.
If MB can prove that the non-mobile1 oil that the customer was using caused his engine to fail, they can deny warranty according to Magnuson–Moss Warranty Act. By them specifying they are simply warning consumers that warranty will not be denied if you use mobile 1, but, it may be denied if you don't use it. They need only prove the oil caused the failure.
Something that needs to be better explained is the viscosity rating. A 0w20 oil means the oil will be a 0 viscosity when cold. 0w oil is tested at -30C degrees, whereas the 20 part is tested at 100C degrees. Once the engine is at operating temps, the oil has a viscosity of 20. The only time it is a 0w is on a -30C cold start. Any higher temp will have a higher viscosity. BTW, the "w" in the viscosity means "winter", not "weight".