The public apis are what you use to get other people to build stuff that you dont want to build and increase the usage of your application by creating an ecosystem around it. Through the api, you draw in users, partners, entire use cases that are not provided for by your app directly and your app becomes something that is much bigger than what could it have been without an api.
The problem with Twitter was that it had no legitimate monetization for the app itself. It was a zero-interest, investor/vc money fueled growth machine. And even for that purpose, it used that api to great extent to bring a lot of users into the platform and integrate a vast swath of internet to Twitter - from Twitter logins to automatic embeds to entire 3rd party applications that served different subsets of users.
But now that the investors who dumped cash on something that does not have a level of monetization and revenue compared to its over-inflated valuation want something for their money, suddenly growth is not that important anymore and problems ensue.
Even in this particular situation, its a dumb idea to restrict or close down an api. If you do that, another service that doesnt do it will get an ecosystem built around it and it will eventually eat your lunch. A fixed set of people working on a singular app in a company can never produce as much features as an entire ecosystem with its large community can produce through an api. The Open Source movement and its successes follow the same pattern: Centralized, large corporations cannot compete with the development speed and breadth of communities of millions of people, even if those corporations employ tens of thousands of engineers.