Giving Notice

I wonder if a employee is expected to give 2 weeks notice before he leaves his job, do the employees have the right to the same expectation of their employer. If you change your terms it's expected but what if they change your terms of employment. Now I don't mean that if you are going to be fired that they should come up to you and say you suck and I'd just like to let you know that in two weeks your FIRED!!!! But what if the company is to be sold and there is no job guarantees shouldn't the employer let you know that a change is coming. I'm pretty sure that Benjamin Franklin said give me liberty or give me unemployment insurance.