The developer should cover for the actions of Twitter because they decided to build their business on top of Twitter, fully reliant on Twitter's goodwill around API access. They are the one who took on that risk, and it's irresponsible and unethical to push the consequences of that risk onto their own customers.

Their average customer doesn't know or care about API platforms. They just want to access Twitter via a nice, premium app they paid for. If they can no longer do that, they deserve a refund of the remaining unused value of their subscription.