It just means "If you do not mark your work as copyrighted using this symbol, and later sue someone for copyright infringement, you will likely be awarded lesser damages. This is known as the “innocent infringement” defence."
The burden of proof lies on the defendant and one criteria is to show that there was no profit motive (under US civil law). Those who can afford to hire lawyer rarely can show no profit motive.
No profit motive violations are not usually the issue.