Yes, they release it under GPL/LGPL and make their money on proprietary licenses for users that can't or won't use those licenses. If you want to develop Qt itself and get your changes included you have to sign a copyright assignment agreement that gives Digia the right to re-license your work. See:

    http://qt-project.org/doc/qt-5/licensing.html
    http://qt-project.org/legal.html