Don't forget that mIRC is just one (and by far the most common) client used to access IRC. There are others (sorry, I can't remember them, I've been IRC-inactive too long) which are popular because users like their GUI's better, or they seem to handle graphics better, etc.
So it's not surprising that mIRC is the one specifically protected by the IM provider. It's a lot like the P2P field, where there are all kinds of services and avast! is constantly adding to the ones which it protects.