When you get a penny for your thoughts, the person wants to listen for free. But when you put your two cents in, you have to pay them twice as much to listen to you yammer.
Because people are willing to pay a penny for my thoughts when they request them. When I impose them on another person, unsolicited, I have to pay them two cents for the time and energy it takes them to listen to me because they might have had something better to do. Seems fair. :-)