I'm surprised at the amount of people just learning about this. Intellectual property rules are insanely pro-corporation. Even your thoughts and likeness (via Twitter/social media) are defined to be images of the company you work for, and are sought to be controlled by that company. Say something silly or do something that raises eyebrows at any point during your employment, and you can find yourself fired for it, even if it has nothing to do with your professional work. That's why sometimes you'll see people with disclaimers that state "All opinions are their own."
All of it is solely for the ability to make more money. Just look Walk Disney's issues with Oswald the Lucky Rabbit or the fact that Conan can no longer use all his classic Late Night characters and bits like In The Year 2000, Triumph the Insult Comic Dog, or The Masturbating Bear.
Moral of the story: Never associate yourself with your place of employment on social media, and if you want to be creative, keep your work secret.