There are certain uses of copyrighted materials that aren't infringement. Those exempted uses are very narrow, and almost universally misunderstood.
First, fair use only applies if the reason for the copying is criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research. If it applies, then a court will look at
- The purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
- the nature of the copyrighted work;
- the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
- the effect of the use upon the potential market for or value of the copyrighted work.
So, saying something is fair use because you've identified the (presumed) copyright owner isn't going to save you. Similarly, using copyrighted material to create something new which you give away (like a YouTube mashup) doesn't mean you have the rights to do it.
In short, it's not always an easy test to pass. I wouldn't suggest anyone rely on their determination of fair use without a legal opinion.