This would make sense if Facebook's business model was selling communications services to people, in which case people would just pay and indeed it wouldn't really matter whether they're running an iPhone or something else.
The problem is that Facebook's business model is to sell out their userbase to the highest bidder, and the bids are significantly higher for iOS users who are considered good marks for ads compared to some generic feature-phone users.
The problem is that Facebook's business model is to sell out their userbase to the highest bidder, and the bids are significantly higher for iOS users who are considered good marks for ads compared to some generic feature-phone users.