Because Bill Gates realizes that China is the world's largest consumer of energy, and its share of total global energy consumption will continue to rise in the coming decades. It is the biggest and most lucrative market for a nuclear technology startup's products and services. (Comparatively, the U.S. is very slow and/or unwilling to invest heavily in nuclear power).

On a "greater good" level: China's enormous fossil fuel appetite can have disastrous environmental and economic consequences, and the depletion of resources could lead to regional or even global conflicts -- whether cold or hot -- in the not-too-distant future. (Many wars are, at their most basic level, contests over the division of scarce resources). So, if we don't help China develop alternatives to fossil fuels, the world loses in a big way. Oil prices spike, economies reel, and the environment takes it in the backside.

I'm not naive enough to assume that the "greater good" argument is at the heart of what Bill Gates is doing. It's not. It's a nice consequence, but the goal here is profit. And it stands to reason that Gates and his startup will go where the profit is. If the U.S. was the buyer, they'd just as soon sell to the U.S.

Finally, the headline of the article is a bit sensationalist. The way it's phrased, it gives the immediate, emotional impression that Gates is selling or developing nuclear weapons technology to China. That is obviously not the case, but it's what many people would infer, even if subconsciously, when they see the phrase "selling nuclear technology to China."