In every country, citizens have more rights than non-citizens. The right to freely enter the country, the right to vote, the right to various social services, etc.

In the US, one of the rights citizens have is the right against "unreasonable searches and seizures", established in the Fourth Amendment. That has been interpreted by the Supreme Court to include mass surveillance and to apply to citizens and people geographically located within US borders.

That doesn't apply that to non-citizens outside the US, simply because the US Constitution doesn't require it to.

I'm not defending this, just explaining why it's different.

But, you can imagine, for example, why in wartime, you'd certainly want to engage in as much mass surveillance against an enemy country as possible. And even when you're not in wartime, countries spy on other countries to try to avoid unexpected attacks.