If spotify started to demand money for artists rather than pay them, artists would start to get angry.
Let's not pretend Eat24 was doing Facebook a favour here, or that anyone signed up to Facebook to look at pictures of pizza. Facebook was doing Eat24 a favour, and Eat24 were earning so nicely from it they were willing to experiment with shelling out money to continue distributing the same content.
That content becomes part of the reason people spend time on the platform, adding more opportunities for Facebook to sell actual ads (including to competitors, I might add). The network effect applies to companies, too!
Facebook is now denying that content to users who have said they want it, and destroying most of the value to the companies of creating that content. This is a reversal of their prior policies (thus a bait-and-switch), dishonest (if they don't want companies participating in their social network, they should just ban them and make them run ordinary ads), and just plain ol' stupid (because they're deliberately damaging their network and their image).
The new Facebook algorithm is kind of like signing up (liking) to a newsletter and then only receiving it every 10th edition.