First of all I appreciate you asking. And I also saw your other thread where you recommended someone to your site. I'd rather people not say "visit my site" but in your case, the person was asking a question relevant to your niche/topic. So I see what you mean.
BUT there are a lot of people here with sites related to Webmasters and web development so if I let everyone recommend their sites in related threads, things could get out of hand.
It's better for me to say "self promotion is not allowed period" rather than giving instances when it's acceptable. Then you start getting into a whole lot of gray areas and confusion.
I have to say, I don't mind it as much in your instance where the the thread was directly related to your site
BUT I have to also be careful how other members perceive what someone does. I know it's tempting to self promote but if you just reply to a thread and provide useful advice people will eventually notice your sig and check it out.
Just understand I have to look out for the board as a whole. And if I start giving instances where it's OK to say "visit my site" then that can open up a great big can of worms. And I don't like worms.