Do you also sell something people do not need? And does it make you uncomfortable when people who do not need it mentions that to an audience via HN, the same way 37Signals keeps working to make mention of itself, or have others do so, in publications with large audiences.
If I have no need for a service someone is selling and I never hear about that service, then I have no reason (maybe even no ability) to to state what I think about that service. But if in the course of reading the "news" I am continually reminded of the service's existence, I may be inclined to comment. That is the risk you take with PR and advertising, or internet-based hype in general.
So it seems that 37Signals sells the use of their forum software (constructed from Ruby on Rails, i.e., free software), hosted on 37Signals' computers. I am getting a clearer picture now.