The difference, I think, is that the former are platforms to run applications while the latter are communication platforms. There used to be a time when the phone company could press charges if you connected your "app" (a tone-generating machine) to their "API" (the phone line) to "do things the official app didn't support" (place long distance calls for free). It will always be beneficial to someone who owns a communication channel to assume as much control of the pipe as possible. If Twitter could jack directly into your brain stem and refuse to work until you install its client on your wetware, it would.