Even though you are entitled to fire people in the US, there are a lot of differing state and city rules. You are also open to (potential) lawsuits at all times. There are also now a lot of very obtuse rules about things like healthcare coverage for full time employees.

There's an extra layer of simplicity when it comes to probationary periods.

In the US you can be fired with 0 notice or severance, regardless of tenure in most states.