Hm. It seems the formatting is quirky. Running this is likely the best way to install:

    composer global require drush/drush:'~6.0'

So now it puts `~6.0` as the version constraint, limiting updates from the 6.x series. If installed that way, then this should be the update process (as alluded in your other comment):

    composer global update drush/drush

The above command does nothing it there's no constraint operator, since the `composer.json` locks to a specific version:

    composer global require drush/drush:6.0.0