Unless all (or enough that there's less open jobs than there's employees) companies require such terms, in which case the employees will either agree to them or starve to death. It is good to have a monopoly on employment >:].

Technically this is known as a MONOPSONY. See the wiki article for a detailed description.

"In economics, a monopsony is a market form with only one buyer, called "monopsonist", facing many sellers. It is an instance of imperfect competition, symmetrical to the case of a monopoly, in which there is only one seller facing many buyers....."http://en.wikipedia.org/wiki/Monopsony