Since the late 60s there has been explicit federal legislation. I assume you meant this as a rhetorical question but there is a definite answer because the US is a transparent system. Law is passed, the courts enforce.

The fact that you are unable to answer this question about China where it is often unclear why certain people are being targeted should demonstrate how big the gap is. For example, when Xi's first corruption crackdown happened, it turned out that it was being orchestrated in part by someone in their 90s who had left front-line politics twenty years ago (and much of why that happened is unclear, we are only just learning about things that happened in Chinese politics multiple decades ago so it will be a while before we know...we do know that Xi was then able to make unilateral sweeping changes to his own role shortly after that broke every convention of the last 5 decades)...it is difficult to compare this to anything that happens in any other political system. In China, you find out someone has been executed for reasons that are obviously not explicit months after it has happened.

It is genuinely quite difficult to compare to anything else. 6 people meet in a room, they have almost all the power, and maybe you will read about what they might have said 4 decades later in a book...that will never be published in China.

The inability to conceive of a country where these things would happen and you would have no idea that it had ever happened...and, perhaps more importantly, wouldn't care that you didn't know.