At any given moment there's probably a vanishingly small number of HN news readers who are unfamiliar with who 37Signals are. So I suspect that for them, gaining the front page on HN is less profitable than gaining traction in nearly any other community. As such, writing interesting stuff that is appealing to a broad base of communities is more profitable for them than tailoring their posts to get on the HN front page.

My supposition; but really, what ethical difference does it make? If the HN community likes to read the 37Signals viral marketing material then I suppose we all gain, right?