In the USA, 'permanent' employees not covered by a union agreement are generally 'at will', which means the employment can begin and end for any reason, or no reason at all -- except for discriminatory reasons, based on certain characteristics listed in various laws.

The factors that can't be considered are 'protected classes':

http://en.wikipedia.org/wiki/Protected_class

So when a decision based on one of these classes could be alleged, employment changes/terminations can become riskier.

There's no right to continuing employment, or direct severance/buyout payments, as seems to be the case in some European jurisdictions. But when more of your employees wind up collecting state-administered unemployment compensation, your payments into that program escalate. (Anyone not fired for specific misdeeds is eligible for unemployment compensation, for a limited period, while actively seeking other work.)