[redthrowaway]

Part of it is almost certainly due to fear of reprisal, but consider also that many people of Chinese descent simply may not agree with the letter. I grew up in a majority-Chinese neighbourhood, and one thing you come to realize is that while many Chinese expats have very complicated feelings about the Party, they still tend to be intensely nationalistic and are very sensitive towards perceived slights directed at China.

They may not want to sign the letter simply because they're offended by singling out China for opprobrium.