It's just part of their business model, that's how they stand out from the crowd. Otherwise how do you think yet another to-do list would be a viable business app. Same thing holds for Joel Spolsky and Fog Creek. Yet another bug tracker. It wouldn't go anywhere without Joel's writings on how to do software.
37 Signals is one of the more annoying Web 2.0 companies. Yes, I know you're trying to drum up interest so people invite you to hold talks, but why do all articles always seem to be about how wonderful you are, how great your company is, and you reinvented something or the other.
Get over it, you guys are not in possesion of the fountain of wisdom.