The FSF may believe that copyleft is the best way to achieve their aim, but they will not actively oppose any other licenses which are compatible. They see proprietary software and its ilk as the opposing force, not anyone else. The FSF looks at MIT and BSD licenses, etc. and think, “Oh, those are some people who might in theory be doing more to support us, but at least they’re not opposing us in any way.”.

Some, even many, people has somehow got it into their heads that the FSF is equally opposed to anything which is not copylefted, and this is decidedly not the case.