If these people are so terrible then why are they going out of their way to annoy the ones who have left the country and may possible help their relatives leave the country?
Do they think that if they prosecute them enough the West will accept them as refugees? (I don't see that happening post-Syria or being good for China if it did.)
In Machiavellian power politics (which is how the Communist party operates), you want to keep all potential usurpers under your control. If any can escape, they will, and they will come after you. This is China reminding them that even though they are not in China, they are not free from China’s influence.
And make no mistake, the folks they’re doing this to are essentially the “nobility” or “landed gentry” of Chinese society. These people have connections and skills such that they will be fine if China does not want them, and they would likely be among the political leaders / influencers in the event of a collapse of single-party rule.
Do they think that if they prosecute them enough the West will accept them as refugees? (I don't see that happening post-Syria or being good for China if it did.)