Most proponents of free software would argue that the difference between free software and open source is a philosophical one, not a technical one [0]. Copyleft is preferable, but not mandatory, for a free software license; the most widely used non-copyleft free software licenses are recognized by such by the FSF [1].

[0] http://www.gnu.org/philosophy/open-source-misses-the-point.h...

[1] http://www.gnu.org/licenses/license-list.html