Why do people feel it is OK to go about posting unsourced quotes on the internet? You'd think they'd want to identify the source, at least to satisfy their own curiosity.

Not only is this practice disruptive. It is also very disrespectful towards their intended readers. It tells them that they owe it to the poster to give them the benefit of a doubt and that they don't deserve the right to look into the information that is being presented.