As for his interest in BYD, it's because the company happens to be a leading Chinese producer of electric cars. The company currently offers three pure electric models, the e5, e6, and Qin EV300, as well as a number of hybrids. The e6 even reportedly underwent testing with Uber. BYD also sells electric buses, some of which are produced in the US. And DiCaprio isn't the only famous American to get involved with the company. Warren Buffet has been an investor since 2008.
So really, it's not that surprising DiCaprio would be willing to do some promotional work for the company In fact, he said, "I am very pleased to be joining an effort to increase the number clean vehicles on the road in China and beyond." We wouldn't expect him to trade in his Fisker for a BYD anytime soon, though.