Some people take importance of good manners and behaviors because they respect others and understand well to treat others the way they want to be treated.

Some people take importance of good manners and behaviors because they're too selfish to do anything that might do harm on their reputation; thus do everything that will paint a great light on themselves. They do it because they seek judgments and compliments, they do it for their own sake instead of others'.

When they sense a place where their ego is comfortably protected, one of which through animosity provided by the internet, they're showing what they're made of. What behaviors? What manners? What civilized discussion? What respect?
Internet does a very good job in showing true colors of some people - giving the luxury of hiding in animosity, in random usernames and untracked identity, so you won't hold responsibility to any useless spouts you're throwing around. So when you feel like being an asshole, you'll just go to some sites and be one; when you feel angry you log onto something and wholeheartedly share your anger in the least civilized manner possible.... and you still can walk around on the streets, mingle with (real) friends, just usual day in the office. No harm done.
When those social-obligations no longer serves their ego right, they soon abandon them and just go follow their own wants and needs - couldn't care less about the receiving end of their own behaviors.

It's very hard to appreciate and respect those kind of people.