Presumably because the firstboot disabling is in the 'livecd' initscript, which only runs in runlevel 5.
Should be "fixed" with http://gitweb.freedesktop.org/?p=users/david/pilgrim.git;a=commitdiff;h=460cdba89a6738e6e5206b97f923af38c0aef88c Spinning a new live cd tonight.