Logback is dual-licensed under EPL v1.0 and the LGPL 2.1, as per the licensee's choosing [1]. In particular, it is not licensed under the GPL.

[1] http://logback.qos.ch/license.html

It scares away people who see 'GPL' and refuse to read further, perhaps. Not only is it dual licensed, but both licenses (EPL and LGPL) freely allow linking with proprietary software.