(edit below)
it has to do with the amount of value inherent to the value proposition. you can try to exact more value from a product and potentially degrade the experience as a result. that's legal because it's a competitive business decision, but at some point you misrepresent the value of the product you are offering my changing the experience too much.
its a hazy line to draw because I think in the OP, the user may not have minded a couple ads, but rather they were annoyed by the inundation of too many ads to the point where you can't experience the actual value proposition of the article. then it becomes "spam".
it is not sufficient to deem this as just another business practice. there are obviously "working" business models such as Ponzi schemes that are highly illegal.