Most APIs are not restful because their designers had more important issues - Would you say the same for most web pages breaking web stanards or is that another matter entirely?
It's more like using tables instead of CSS -- the only real compliance with web standards here is valid vs. invalid use of HTTP; everything layered on top of that is mostly subjective in the case of REST dogma. In fact, some of the things in this article directly contradict recommendations in O'Reilly's RESTful Web Services.
First author of HTTP 1.1 is Roy Fielding, same guy who defined REST. In fact, http 1.1 was designed largely to make HTTP restful. As such, you'd be surprise how far HTTP goes towards RESTfulness. For instance the spec defines idempotency and satfety of methods. Ergo, servers may produce valid requests and responses but still fail when it comes to being valid http if their internal operation does not conform to the operation semantics. Look up the google web accelerator fiasco to see how things can go terribly wrong.
To be clear, the Google Web Accelerator fiasco was not related to anything Google did wrong with Web Accelerator. It was a result of other people doing things wrong.