It's not arbitrary and it's not just CO2. The Euro 6 and upcoming 7 standards also test for carbon monoxide, unburnt hydrocarbons, nitrogen oxide, and particulates from the tailpipe, brakes, and tires. The Jimny couldn't pass on those and Suzuki decided it wasn't worth the price of compliance to add things like gas particulate filters and change the brake pad material.
Mercedes does not make a G600. If you're not talking about Mercedes then you should specify a manufacturer because I can't figure out what car you mean.