How it worked until now:
1) IFTTT wrote glue code to interface with a site's published API, and IFTTT could then write recipes to use the site. IFTTT thus stitched together other sites published APIs as a service.
How IFTTT has decided it will work from now on:
1) IFTTT comes to you and orders you to write private endpoint(s) that do X, Y, and Z. They reserve the right to ask for additional endpoints, different call results, or anything else they dream up down the road. You sign a legal agreement promising to ask "how high?" any time they say "jump". You will not be compensated for this privilege.
Later, after putting a bizarre legal agreement in front of him to sign, IFTTT announced that Pinboard would stop working on their service in a manner that made it sound an awful lot like Maciej had just decided to stop supporting IFTTT. In reality, all the agency in this relationship belonged to IFTTT.
It's essentially the "protection/promotion" model.
Many business owners have claimed yelp salespeople have called them and tried to sell them the ability to bring down unfavorable reviews or that they would highlight them.
One google search away: yelp wins lawsuit that says they are perfectly within their rights to manipulate their ratings: http://www.dailydot.com/opinion/yelp-extortion-lawsuit/
"Yelp is pay-for-play, and the court says that’s just fine."
The ruling was that if yelp manipulated ratings, that's fine, but there's no evidence they did. I don't know whether to believe they've done it or not. There's a lot of business owners over the years who report having gotten shady calls, but it'd be really hard to prove those calls are from yelp and not from fly-by-night scammers. I get calls from "Cardholder Services" and "Microsoft" pretty regularly, so it shouldn't be surprising if there are extortive yelp scammers.
That is surely how IFTTT will pitch it to 3rd parties... but, really, probably not. Being accessible in the Recipe list isn't great advertising, anyone not using your site already probably isn't going to sign up due to seeing your name on IFTTT.
Framed another way, there is a famous saying in the design industry: "I can't pay you, but it would be great for your portfolio". Classic exploitation tactic; manipulative and predatory.
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).