If the US has laws that forbid that, and other nations have laws that establish that, then the US military being used for police activities is threatening to democracy - or at least to the rule of law - in a way that it is not threatening in other countries.

Other countries can do that if they want. It may or may not be a threat to them. But in the US, it's absolutely a threat to democracy, because it's already the executive deploying the military against the law.