The anonymity of the internet often makes one assume the inferiority of others, particularly when interaction is purely online.
In this light, many lose their sense of propriety, often assuming the other person, with whom they are in interaction, to be a peer or a subordinate.
Losing your manners under such assumptions cast a very long shadow that’s difficult to shake off, even after you might establish contact in person. Simply put, politeness can find no surrogate when you interact with others online.