Twitter set the precedent, and Reddit is just being a fast follower since they've got their IPO to consider. The people doing this are just looking to make a quick buck and bail, and they could care less about the long term health of the platform.
Struggling businesses don't like it when third parties are able to successfully monetize off of their content. It's their product and they feel entitled to the profit that is being made off of them. (understandable to an extent) Up until now third-party apps have been seen as a "necessary evil" as a submission vector for more content, but now the AI fad has blown up and these sites are all the more covetous of how the content they host is being used to rake in profit. The value proposition has shifted and now they are more interested in capturing income from what is consuming them through the API rather than the content that is being produced through it. The fact that their first-party apps are able to better monetize the user interactions is the icing on the cake.