Quote from Sven Wegener: It allows us to force some USE flags to be turned on for a profile. It's not possible to disable this flag by make.conf, the environment or package.use. But we would not be Gentoo, if we don't leave a backdoor. You can disable the flag by putting -flag in /etc/portage/profile/use.force if you really need to. Same goes for sub-profiles that need to disable this flag. Full discussion: http://thread.gmane.org/gmane.linux.gentoo.devel/28727
This is in svn r4214 and has been released in 2.1.1_pre5.