They want you to write your app so that all of your code is so hopelessly coupled to their service and API that it may as well just be an extension of it.
I had a similar revelation when I was writing an S3 backup system recently, I was writing hundreds of lines of Java that was catching exceptions that I hadn't even heard of while calling libraries in the namespace of a company that is known for selling books. What on earth was I doing?
So I wrote a 3 line bash script to rsync to a bunch of places and added it to crontab. Now that means I can change backup providers whenever I want with a single code change.
As a separate point there is something to be said for stateful protocol (especially open ones) instead of stateless by design API stuff like REST.
I am considering the use of an IRC server as a backend for a messaging/logging system.
I like building systems that can be debugged with telnet.