mail-client/thunderbird-bin does not install enigmail while mail-client/thunderbird does (with crypt flag). I'm not sure whether this feature for ebuild is necessary (as enigmail can be installed through thunderbird addons menu), but as there is such a possibility in mail-client/thunderbird ebuild it may be also usefull for a binary one.
If your using the -bin you need to install it yourself. The reason it is included in source build is so it can be used on archs not officially supported upstream.