To stop companies from using social media sites, such as Twitter and Facebook, to judge whether or not to hire or fire someone that posts about anything the company disagrees with, effectively censoring our freedom of speech when away from the workplace and "off the clock". This will only lead to people being afraid to say, think, share, or do anything out of fear of not getting or losing a job. This is no different than if your employer (or potential employer) followed you home and didn't hire you, or fired you, because you went to a bar for a drink, or watched a movie they didn't agree with. It is none of their business what we do in our personal, private lives away from the job.
It IS a form of discrimination as well as spying and web stalking