Firing people is not exactly hard in the USA.

It's perhaps not hard but the expectation if you take a new job is that you won't be randomly fired in a few months because you don't meet some unstated high bar.

Every new hire has a probationary period to it. If you go into a new job thinking you'll make it past 6 months automatically, then that's a big assumption. While it shouldn't be hard to just do your job and get past the probation period, it's hard to know if the company didn't just hire a bunch of people to get past a crunch without an intention of keeping them.

Implicitly. But there's a difference between "wow, you're a bad hire" and "you're not an 80%er."

"Does not meet expectations" is always a cop out, but a great catch-all if aren't an 80%er.