Forgive my lack of knowledge, but is this just a US thing? It seems that you can be let go (from a permanent position; we'll not talk about contractors/casuals) very easily in the US, without much reasoning.
To contrast, here in Australia, a part-time/full-time employee needs to be formally warned (and these need to be documented) three times before being 'fired', and there are many cases where Fair Work (a government body) has helped an employee take an employer to court in cases where the documentation was insufficient.
Of course, that's a very high-level overview, but the message I constantly get is that you can be fired for really simple reasons in the US, without much recourse (lest you want to take it on yourself).