Since bug 318897, emerge --buildpkg maps to FEATURES=buildpkg, in order to support FEATURES=buildpkg settings from package.env: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=c47394cb28a841d5b7399f548c2507a85a158bdd It would also be nice if emerge --buildpkg=n would map to FEATURES=-buildpkg.
This is fixed in git: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=500e0567432019835ef56e69d60266f9189c6700
This is fixed in 2.1.10.50 and 2.2.0_alpha94.