Most forums have a rule regarding that, ours is:
[9] Threads are not permitted for promotion of sites similar to Ausphotography. Blatant, direct, promotion of other photographic forums is not allowed. Comment or reference to another site as part of an ongoing discussion in a thread is permitted, provided that the thread or post is not blatantly promoting the other site and is in context with the thread topic and discussion. This also includes using the personal messaging (PM) system to promote other sites.
But other sites differ in how they word their rule and interpret it. We will not remove a link if it is relevant to the discussion and thread topic, but we have had people post basically asking members to join another forum, etc, and those we remove. We have also acted a couple of times when specific members have repeatedly linked to another forum. It isn't so much about sites being a business (I could not live off what AP makes, and all the funds are directed back to the site anyway), but rather about people trying to 'steal' members from one site for another. It is just not good etiquette. It would be no different to me coming to your workplace, standing out the front with a big sign, and telling everyone to come down the street to my business instead.