[kh99]

Well, if you look at file includes/functions_newpost.php around line 1192 you'll find this:

		WHERE subscribethread.threadid = $threadid AND
			subscribethread.emailupdate IN (1, 4) AND
			subscribethread.canview = 1 AND
			" . ($userid ? "CONCAT(' ', IF(usertextfield.ignorelist IS NULL, '', usertextfield.ignorelist), ' ') NOT LIKE '% " . intval($userid) . " %' AND" : '') . "
			user.usergroupid <> 3 AND
			user.userid <> " . intval($userid) . " AND
			user.lastactivity >= " . intval($lastposttime['dateline']) . " AND
			(usergroup.genericoptions & " . $vbulletin->bf_ugp_genericoptions['isnotbannedgroup'] . ")
	");

The line is red says only select the user if they have visited since the previous post. If you took that out, everyone would get a notice for every post in threads they're subscribed to (even if they are logged in when the post is made). If you wanted it to be a setting, you'd have to do something more complicated of course.