The following behavior was reported as corrected in ticket #12065, but still exists in v3.0.4:

On a site where account activation by an administrator is required, users are informed upon registering that they will receive an email once their account has been activated. This all works out very nicely if the admin activates the user's account by clicking the link given to him in the "Activate user account" email. However, if the admin activates the user through the inactive users section of the ACP, the "account activated" email is not sent to the user.