Not all free software licences are copyleft licenses. While the GNU GPL is a very well known and widely used example, it is by far not the only free software licence. The Apache 2.0, MIT, BSD and other commonly used licenses are free software licenses because they provide the recipient the four essential freedoms. Copyleft is not mentioned in the free software definition[1]. It's mentioned as an optional way to license free software works.

In addition, even with copyleft every contributor has copyright over their changes. The crucial part of the copyleft hack is that it requires that distributing derived software must be done under the same license. You may choose to not distribute the software, and then the GNU GPL doesn't require you to do anything.

[1]: https://www.gnu.org/philosophy/free-sw.en.html