I responded to this sentiment in another comment, but I'll just quote from the HN guidelines here: "Please respond to the strongest plausible interpretation of what someone says, not a weaker one that's easier to criticize. Assume good faith."[1]
> How do I know which emails I can trust as having useful unbiased information (i.e. satisfying my needs), and which are potentially misleading junk adverts (i.e. satisfying the site operator's desire for profit)?
Because ads are clearly marked with a "sponsored" label.
> And what is to stop the signal to noise ratio becoming completely untenable over time in the desire to maximise profit?
Because if we send people newsletters they don't like, they'll be more likely to stop using the service.
> the conflict of interest between user needs and site operator's profit.
Interesting anecdote: I haven't measured this in a while, but at least when we first introduced ads, the sponsored newsletters tended to be rated more highly than the "organic" newsletters.