It's not (per Apple). It's an appliance, and they use that as guidance in the decisions they make. It may be capable of acting like a computer, technically it obviously IS a computer, but Apple treats it as an appliance.
I think the relevant distinction between iOS and OS X is that, with iOS, there is sort of an understanding that the user is granted less direct control, in exchange for a near-guarantee that they won’t have to worry about certain things (like malware, or to a certain extent, battery life) anymore.
But it's pretty clear to me that that Apple is not selling the iPhone as a general purpose computer, and that's fine with me.
Android has allowed things (widgets, alternate keyboards, different launchers/lock screens, background processes) that expose more power but rely on the 3rd party developer to ensure the experience still works well without wasting resources or crashing.
Apple only provides such things after careful study and lockdown (such as keyboards). Much more careful, much more sealed up unmodifiable box.