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).
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:
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.
Well, if we're talking copies, then Airs are just a scaled-up version of the eeepc - same 'sharp wedge' shape (that is apparently why Lenovo X1s are being called clones of Airs).
The eeepc was the thing that proved there was enough interest for a low-power-but-mobile computer, which ended up turning into the tablet market. Crappy build quality, but people went berserk for them.
And how!!! My ex-boss read the press announcement on a flight (sometime in 2011?) and went gung ho. On coming back, he got me to inquire with our regular/local hardware vendors. Turned out nobody (in our small city) had even heard of it, so the inquiry was escalated to the largest city of our state. Still nothing. Finally, he actually got a staffer to fly to Mumbai and get him a piece.
Within the next few weeks, he bought not less than 7 more, to gift them away to family and friends!! Yes, berserk is right. :-)
Xiaomi is a Chinese company they don't care since if they ever do get sued the Chinese court will always side with the Chinese company.
The again if the clone works just as well as the original I don't really care but this would have been a really good chance to copy and improve.