What in particular do you think makes them vulnerable to a lawsuit? I am not a lawyer, but my understanding is that in the US, it is legal to fire most non-contract employees at any time, without any notice, and for pretty much any reason (or even lack of a reason) other than outright discrimination based on nationality/gender/religion/etc.
See http://en.wikipedia.org/wiki/At-will_employment and specifically the "Statutory exceptions" section.