Except the Mini brand predates BMW ownership by 25 years and even the classic Mini is labeled as a German car; and the logic still doesn't follow anyway because only one of the other brands in the game owned by a larger brand has the same country as the larger brand.
Scion is American despite being owned by Toyota and made up entirely of rebadges.
Infiniti is American despite being owned by Nissan and made up almost entirely of rebadges.
Scion and Infiniti are the US versions of the Japanese brands. MINI is not the British version of BMW. It's not comparable.
If you want to discuss the nationality of Scion and Infiniti, that's a different discussion and is not related to Mini.
Bugatti is French despite being owned by Volkswagen and with no illusions about the car having any French design in it.
SEAT is Spanish despite being owned by Volkswagen and generally only making hotter versions of slightly older VW hatchbacks.
Bentley is British despite being owned by Volkswagen and the one car in the game being built off of an Audi race car and ran by the Audi factory racing team.
Land Rover is British despite the only car in the game being designed and funded while under Ford's tenure and the company being owned by an Indian car company by the time GT5 came out.
Opel is German and Vauxhall is British despite the two brands being so identical that they even copied cars over between the two that they shouldn't have; and despite both being nothing more than European arms of GM.
MG is British despite the fact that (by the time that GT5 came out) it was owned and run by a Chinese car company who were building the only car in the game (the MGF/MG TF) entirely in China and having them shipped to Britain for reassembly as knock-down kits.
Bugatti is a manufacturer. They make cars and they are owned by Volkswagen.
SEAT is a manufacturer. They make cars and are owned by VW.
Bentley is a manufacturer. They make cars and are owned by VW.
Land Rover is a manufacturer. They make cars and are owned by Tata Motors.
Opel and Vauxhall are both manufacturers. They make cars and are owned by GM.
MG is a manufacturer. They make cars and are owned by SAIC.
Mini is not, and has never been, a manufacturer. They do not make cars. Mini has never been more than a marque that's been owned and manufactured by other companies.
But Mini is German because BMW bought out the (British) parent company and broke most of it up but continued making and designing the brands they did keep in Britain in the same 100 year-old factory (remodeled and updated) that most of British Leyland's output was made in?
Mini never had a parent company. Why? Because Mini was never a company, Mini was the intellectual property of Rover, by the time Rover was acquired by BMW. Whoever bought Mini bought the name and the design of a car - there was never a company to go with it. When BMW decided to sell off Rover, they kept Mini. Mini is now the intellectual property of BMW, and BMW decided to use it on their new segment of compact front wheel drive cars.
As a result, the MINI is a car that is manufactured by BMW, it's a German make.