(from outside of US) The article looks like written on the assumption that every Internet user in US is automatically an infringer. Are copyright infringement so popular there?
One of the details that this stipulates is that works are automatically copyrighted upon creation – even if you don't register work with the copyright office it still holds protection.
This means that any picture, text, or other work in a tangible medium that you share without permission could be considered potentially infringing, and leave you open to a lawsuit.
There are defenses you can argue, but that would only be after the lawsuit has already started, potentially costing you time and money for legal representation.
Are people share the things without permission there, so frequently? Even if so, why should these sharing be legal?