Not true. "can claim" means nothing and "did not know" is not valid defense. Copyright is well protected without any symbols or copyright notice. Content is automatically protected by copyright. With, or without copyright mark you need to fight the one who uses your content without permission.
If you want to make claiming copyright easier, register your unpublished or published work. In US you can do it in U.S. Copyright Office
In EU you can use Datasure in Copyright.eu to get official proof of the date and content of your creations.
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."
I don't think it's "likely" but sometimes happens.
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.
The difference in levels of damage is less to do with the presence of a copyright symbol and more to do with copyright registration. Registered copyrights are eligible for statutory and actual damages, while unregistered copyrights are only eligible for actual damages.
And those damages can be significant in the US. $750-$30,000 per work. That only applies to US law. Copyright violations are often international nowadays.
If you want to make claiming copyright easier, register your unpublished or published work. In US you can do it in U.S. Copyright Office
In EU you can use Datasure in Copyright.eu to get official proof of the date and content of your creations.
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.