The point is that "regime" is used when someone wants to depose it. And the US government is a loud voice advocating for deposing governments, so its preference

The point is that "regime" is used when someone wants to depose it. And the US government is a loud voice advocating for deposing governments, so its preference guides the commmon use of language. A clear example is Israel: it has a "regime" in Arab and leftists human rights communitjes, but not in US mainstream media. China has a "regime" depending on how hard line anti-Communist you are. Even though the government has no democratic legitimacy (like the Soviet regime in the cold war), it's not a "regime" in diplomatic circles because the US is economically dependent on China. Likewise Russia is not a regime now that the cold war is over (for now).