Thought experiment: if say Rhode Island had a Muslim majority and they voted themselves Sharia law in violation of federal articles, how cavalier would the federal government / the rest of the populous be in tolerating this?
However, it would be a mistake to think of the Hong Kong matter in this way. To quote the Beijing party line, "Xi Jinping thought on diplomacy has made innovations on and transcended the traditional Western theories of international relations for the past 300 years" (i.e. back to the Treaty of Westphalia). Instead, Xi Jinping thought revives tianxia, the mandate of heaven, and envisions total world domination in a heirarchical system under Chinese rule. "The Chinese have always held that the world is united and all under heaven are one family," Xi told us in his 2017 New Year's message.
Thus, even as China increasingly meddles with other nations' affairs to effect its strategy of world domination, it is naturally unconscionable for other nations to interfere with theirs or attempt containment. It is against this framework that we should consider the matter of Hong Kong.