As a wealthy and influential man - and one who genuinely seems to display a sense of ethics in his investing - Warren Buffett should be free to invest in any legal enterprise. He understands the considerable worth of his time, and the extent to which he decides to involve himself in car sales is almost certainly because he believes there is value to his doing so.
Warren Buffett is a successful businessman who already incorporates other modes of transportation in his company, and the cars should be one of them. He is smart and will most likely improve the sales of car companies and open his company to more and more automotive opportunities while benefiting the auto dealerships as well.
Warren Buffet is a financial guru. He would excel at selling a car, even if the buyer didn't know he/she needed a car. He's very convincing on many levels. Not only would Warren Buffet make an excellent car salesman, he would likely revolutionize the car buying process with his, seemingly, unlimited amount of resources and his vast knowledge.
He played his cards right, invested very wisely, and he is definitely showing good results for it. I don't see why he shouldn't be involved in car sales, he did buy a company that sells cars. If that is what he wants to do with all his money, I have no problem with that.