According to alex4's author, its license (GPLv2 or later) applies only 
to the software, not to any of the data.  The data don't have a license 
specified, and are therefore proprietary.  They should either be 
replaced (if there is a free replacement) or the package removed.

Source:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1035043
https://libregamewiki.org/Talk:Alex_the_Allegator_4