That doesn't mean that they aren't copyrighted. It just means that you can freely use them under the terms of their copyright license. If the code for them were not copyrighted (usually referred to as Public Domain), then you could do whatever you wanted with them-- things like making derivative works and using a commercial license.
Everything You make gets Your copyright by default. This means the default is: nobody can use what You make.
You can put other copyright terms on your works by adding a copyright notice.
You can put something in the Public Domain by adding a notice to your work.
Things can also become Public Domain automatically after a certain amount of time. But that time has been extended several times in the US to something unreasonable: like 75 years after the death of the author. You can thank Disney/Mickey Mouse for that.
