Instapaper’s Marco Arment knows a thing or two about APIs, of course, thanks to creating his landmark app for reading long-form web content later. This expertise makes his theory about this week’s closure of Google Reader all the more interesting. It’s not about the RSS feeds, nor is it really about the money (at this stage). Instead it’s all about the APIs…and specifically locked-down APIs that force netizens to behave in a particular way when using apps.

Arment’s argument runs like this: Reader didn’t cost Google anything to run, and it may have been effectively zero-staffed for years–it was, as he says, “just running, quietly serving a vital role for a lot of people.” And RSS itself, which was “the original web-service API” and “the original mashup enabler,” is by no means dead, as the explosion of RSS readers created to fill Google Reader’s void can attest.

So if it wasn’t the cost of running Reader or any sort of fossilization of its core tech that caused Google to close the system, then why? Is it the money? That’s a theory being espoused online, suggesting that Google couldn’t work out how to make money from Reader so it shut it down instead. This seems specious, because Google runs many services that don’t make it money.

Instead, Arment points us to the new era of APIs that are limited or locked down in some way. The death of the free and fully open API is an inevitable trend, he suggests, citing Twitter’s unpopular decision to shut down some critical APIs and thus kill off third-party Twitter apps. Twitter can’t run its operations on its own cash, and simply let other companies profit from its activity.

And here’s the core point: “While Google did technically “own” Reader and could make some use of the huge amount of news and attention data flowing through it, it conflicted with their far more important Google+ strategy: they need everyone reading and sharing everything through Google+.”

Google is desperate to get everyone using its Google+ social network. Closing Reader may indeed send much web traffic to rival services, but Google hopes that it will also shunt traffic toward Google+, where it can serve to generate more revenue. And Google+ has locked-down APIs to keep you trapped inside its ecosystem (just as Twitter, and Facebook do too). For example, you can’t even write a third-party app that will automatically take a tweet and use it as a status update on G+–so intent is Google on corralling netizens. Keeping them inside Google+’s tightly controlled walls for all purposes, including news reading, could enable the company to make more money.

The lockdown API is something to think about, and it irritates Arment, who has a battle cry for developers everywhere: “We need to … do what we’ve always done before: route around the obstructions and maintain what’s great about the web.”