From the FHS: "/usr/lib includes object files, libraries, and internal binaries that are not intended to be executed directly by users or shell scripts". Some of the programs in /usr/lib/mailman/bin are supposed to be directly executed by the sys-admin, one example is mmsitepass. The mmsitepass program belongs in /usr/sbin (or at least a sym-link so that the sys-admin can run it without doing an "rpm -ql mailman|grep mmsitepass" to find it first).
From the online documentation of Mailman (http://www.gnu.org/software/mailman/site.html): "You must run these scripts from the bin directory in the Mailman installation location...". I agree Mailman is not FHS compliant much. But finding what script under what circumstances can be run from outside the Mailman installation directory is not worth the effort -- the only thing we can get are distro-specific issues. Also note that all the binaries have (and must have) sgid on.