Unlike in many countries, in US in most states employment is "at-will" this sounds nice and friendly but it means officially they can let the employee go for any reason at any time unless that reason is one of discrimination due to a limited set of categories. Of course people are discriminated all the time based on religion, race, sexual preferences, age, gender etc, it is just that is not the official reason. It can always be molded into some "performance review", "downsizing", "business goals" etc. kind of thing. Proving otherwise requires an expensive lawsuit looking for patterns over a longer period of time and candidates or requires someone to blow the whistle and say "oh when the CEO say 'culture fit' we really mean they are of a wrong race or gender, everyone knows that around here..."