The FSF has written extensively on why (in their opinion) you should prefer copyleft licenses over non-copyleft licenses, but they don't require a license to be copyleft in order to be considered free. It's worth spending a bit of time on their site to understand their point of view. Just be careful not to drink too much of the Kool-Aid or you'll become one of those annoying people who never shut up about the GPL on forums.