Amavisd-new doesn't need SpamAssassin to run nor does it need a virus scanner. As such, they should not be installed by the ebuild. In my case, I only want to scan for viruses, not for spam. So I don't want it to install SpamAssassin and all its dependencies. Some other people may want the other way around, scanning for spam but not for viruses.
Fixed in the new release... Bug 56233 for more details. Thanks!