I haven't selected a topic and subscribed to it, but my settings were set so that if I reply to a thread, I automatically am subscribed to it and I was getting email notifications for those updates.
You can change that setting through your UserCP page so you aren't automatically subscribed to threads you respond to. Look under Edit Options.
Edited to add...just tested the general subscription feature, where you select a thread and subscribe to it. It works for me too.