Taiwan isn't China. Completely different legal structure.

And Lenovo is a Chinese company, but I guess they've made the cost-benefit analysis that they have more to lose getting sued by Apple in the US for copying their products than they have to gain by selling a cheap Macbook knockoff in an already very competitive Chinese market.

The difference is that Lenovo and Acer sell to US market whereas Xiaomi does not. This means that they have something to loose. Xiaomi will be in big trouble if they ever try to break into a market outside of China (and other places with weak IP laws).

China and Taiwan are two different countries in practice (despite claims China makes to the contrary).

Taiwan has robust rule of law, unlike china.

Lenovo inherited its entire design style from the IBM sale. A modern thinkpad isn't that different from one from 2003. Lenovo doesn't need to copy anyone. Its designs are already considered classic and unique to their brand.

Xiaomi is the new kid on the block and just copies Apple designs. Some examples:

http://www.iphonehacks.com/2014/07/xiaomi-copying-apple.html

Also they have no legitimate sales channel in the US/EU, so there's a jurisdiction issue with a lawsuit. Apple knows it can't just march into China and win a lawsuit. Acer and Lenovo sell in the US/EU, so they have to play ball with our IP laws.

If you do business together in a country, you have to follow the laws of that country. Xiaomi does no "official" business here and there is nothing for apple to sue.