Individual sites can't measure how much their combined spam costs them due to the ever more aggressive use of ad blockers. In the short term, a newsletter popup probably does increase revenue, but acquiring new users in the future will be more expensive as a result because the cheap and simple ads will fail to reach most of the market. Unfortunately, nobody has incentive to stop, as they'll just give up the short-term revenue and still suffer the future consquences from all of the other bad actors.
