I think you've really missed the point here.
Nobody is arguing that IFTTT can't change their service as they see fit. They can, as they've chosen to here, move from a model in which they write client code for other sites APIs, to a model where other sites write client code for their API. It's obviously beneficial to IFTTT if they can offload this work to other sites for free.
What's shitty and rude and presumptuous about their behaviour is going to other sites who never asked to be part of IFTTT (IFTTT was just a consumer of pinboard's public API) and saying "here sign this and get cracking on implementing our specs".
And when a site owner refuses such a "request", IFTTT is basically lying to its customers when it says that Pinboard has chosen to leave the service, rather than "IFTTT has chosen to shut down their working pinboard API client code".
To reuse your analogy, it's as though Google themselves built an App client for my site, then decided to move to a model where Android apps were responsible for hosting and bandwidth of their own app (their right) and disable all apps in the store who didn't specify a hosting location (their right), deleted the app they built (their right), mailed me the specs for their app and a deadline to reimplement it myself and host it at my cost (why would I be interested in this? I never asked for any of this), and then when I told them to get bent emailed customers "Sadly, beloved app $APP has chosen to take themselves out of the Android store" (lying).