Pretty much the only significant reason you can't get terminated in the US is as a direct result of being in a protected class (so you can't fire someone for being black but you can fire a black person for any other reason including no reason at all)
http://en.m.wikipedia.org/wiki/At-will_employment
Pretty much the only significant reason you can't get terminated in the US is as a direct result of being in a protected class (so you can't fire someone for being black but you can fire a black person for any other reason including no reason at all)