I don't understand what their issue is - you're not selling products to undercut them, you're providing more information.

The ads you see flashing on bb.com every time you use their forum equals money in their pocket, and more money if they show the company the ads are getting a lot of views.

Drawing traffic in that light could be seen as potential profit loss.

Not that I agree with that, but it could be the motivation to disallow outside forum links even without sales.