squid-3.1.10-29.el6.src.rpm (from ftp.redhat.com, buried where they keep SRPMs) has squid.init within, and that file has no mention of SQUID_PIDFILE_DIR. A few other spot-checked versions are the same way.
I guess they applied that change which was obviously written against a very different init script where the variable is actually defined, got QA to test it and immediately backed it out.
That would be my guess too, although the package maintainer confirmed it on (presumably) a fresh install. I can't find a candidate rm command anywhere in the SRPM, so maybe an upstream file got merged into distrib somehow? I don't have access to an RHEL system to try it out, and can't find the distrib RPM yet to check.
https://github.com/mozilla-services/squid-rpm/blob/master/SO... ... however ... whatever that is, does.
Did they take this upstream init script somehow?