The idea behind probation period is that it becomes much more difficult to fire someone after that due to labor laws. US doesn't have such laws, most states are at-will employment so you don't have to give any reason to fire someone.
Probation periods are seen in the US, despite "right to work" labor laws because A) the company is large and has employees in California, or another state where labor laws favor the employees a bit more or B) the company wants to minimize the chance of getting slapped with a wrongful termination lawsuit. Regardless of labor laws, it's universally illegal to discriminate/fire someone for their race, religion, gender, marital status, age, et. al. and I think companies, especially when they want to fire someone who might believe they are being discriminated against, want an airtight defense (usually a PIP). Even though the employer will probably win the legal case, they still would rather not have a lawsuit at all.
