It's pretty simple isn't it? If you were to slag off the company in real life you probably wouldn't stand up in a shopping centre with a microphone and amplifier and proclaim "I work for Qantas and this is why they suck!" Or if you did you'd reasonably expect them to be unhappy with you. Same if you started shouting out stuff that they consider to be commercial in confidence. When you slag the company in real life you probably do it in private to friends and work mates.
Well when you post stuff on social networking sites you are metaphorically shouting from the top of a tall building. The more "friends" you have the more public your ramblings are. Needless to say that if the company find out what you've been saying then you haven't been discrete enough.