In the US I think it's entirely up to the school. Or at least the accreditation bodies in different fields and regions.

When I was in school, one applied to "MS/PhD" programs most typically. And all imaginable variants existed in terms of funding and likelihood of leaving with only a MS. Some schools would fund no one till they finish the MS stage, and others would fund everyone from the beginning (just give every accepted student a full fellowship for the first couple years). And also some schools were pretty notorious for kicking people out at the end of the MS stage, with a difficult exam at this step or not enough advisors to take them on for the PhD.

One positive side, though, is if you wash out of a PhD program at any later stage, you at least walked away with a Master's.

I don't think so. It is funded and recommended, but direct doctorate is also possible, unlike many European schools.