I have been groaning about income inequality a lot but it is amazing how much of this can be explained by it. People do not have the disposable income to spend on services so you make people pay with attention. Give them the carrot for free so they don't notice. On top of that, the product is free so there is no expectation of support for the end user. You're getting it for free so what are you complaining about?
What would be the point of showing an ad to someone without disposable income?
More generally, if the service is free, you're the product, and you're being sold to someone else