Why? Well, amid waffle about the help that the mainland gave Hong Kong during the SARS epidemic (an epidemic that spread to Hong Kong due to the failings of the PRC government), and the benefits that Hong Kong receives from the PRC governments efforts to prevent "foreign forces from interfering in Hong Kong's affairs" (which are?), the white paper dropped this bombshell:
As a unitary state, China's central government has comprehensive jurisdiction over all local administrative regions, including the HKSAR. The high degree of autonomy of HKSAR is not an inherent power, but one that comes solely from the authorization by the central leadership. The high degree of autonomy of the HKSAR is not full autonomy, nor a decentralized power. It is the power to run local affairs as authorized by the central leadership. The high degree of autonomy of HKSAR is subject to the level of the central leadership's authorization.That is, the PRC government wishes to make it known that it does not consider the promise of 50 years without change to be a binding one, but that Hong Kong's autonomy could be removed by the central leadership before that. What could cause them to remove it? Well, the white paper further goes on to state that:
. . . the "two systems" under the "one country" are not on a par with each other. The fact that the mainland, the main body of the country, embraces socialism will not change. With that as the premise, and taking into account the history of Hong Kong and some other regions, capitalism is allowed to stay on a long-term basis. Therefore, a socialist system by the mainland is the prerequisite and guarantee for Hong Kong's practicing capitalism and maintaining its stability and prosperity. For Hong Kong to retain its capitalist system and enjoy a high degree of autonomy with "Hong Kong people governing Hong Kong" according to the Basic Law, it must fully respect the socialist system practiced on the mainland in keeping with the "one country" principle and, in particular, the political system and other systems and principles in practice.This is pretty clearly a threat to the people of Hong Kong from the CCP: don't do anything that might threaten our death-grip on the mainland, otherwise we'll take away whatever freedoms you currently enjoy that are not granted to the rest of China. That this comes at the same time as Occupy Central is preparing demonstrations and unofficial referenda that may be embarrassing to the central government can hardly be a coincidence.
Some critics have attempted to make this out as merely a restatement of long-running government policy. It is nothing of the kind, as even CCP-apologist Lau Nai-keung has to concede, the PRC government has never actually stated anything like this before. Whilst many simply suspected that the PRC government would be willing to abrogate "One Country, Two Systems" if they felt it suited their interests, they have never gone so far as to actually say so.
So where does this leave us? Well, clearly the prospect of Taiwan ever willingly joining mainland China to form a single country under the "One Country, Two Systems" formula is deader than a Dodo for at least as long as the CCP remains in power. Who would ever trust the CCP not to simply withdraw their promise because they felt that people on Taiwan did not "fully respect" the mainland's political system?
"One Country, Two Systems" is an idea that can work, at least in theory, so long as the two systems are on a par with each other. To state openly that they are "not on a par with each other", is to state that one may over-ride the other, which is to state that there is no real guarantee of two systems coexisting.