Why label your company as "remote-first" if you only hire from the United States?
I don't think those are mutually exclusive to be honest. You can have a remote-first company and that could mean there's no offices and anyone works from remotely.
Maybe it's also it's easier (taxes, legal) to hire from the same country you're incorporated in. But you are in a sense limiting your teams by not being open to hire better talent from a global pool of people. It's a decision corporate has to make and it's not easy I'd say.
That being said, job postings should include these facts. I've seen quite a lot of postings labeling the position "Remote (US)" or "Remote (US, EU)" or even just writing the "work time" timezones in the description and that's a good way to be upfront about stuff like this.
It would be beneficial if they were to provide a comprehensive explanation of job requirements in their listings/careers page so applicants from all over the world could get a clear picture of what's possible and decide if it's worth applying.