Calling it the developers fault is just ridiculous.
Twitter should either own up to their current strategy and remove the API altogether (publicly), or keep the API and support it like it's a service that matters.
That's true, but in no way is this invitation a binding contract.
It's a temporary bargain: The big player gets a cheap way to offer more functionality to his users, and the API-users profit from the big audience of the platform.
As soon as one player decides that the bargain isn't worth it anymore, the deal is off. It's no one's fault, just the nature of it.