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.
May I ask why? Both Canada and USA allow employers to fire with 2 weeks (or a month .. I forget )notice.