Your first point is subjective, your second point is confusing (pull to refresh is the only way refreshing works?) and your third point implies that people are already familiar with it (on Android you're mostly not, I've only ever seen it on lazy iOS ports).
iOSey or not, it's quite easy to discover, and is accessible from anywhere within the thing that you're pulling. On a phone as big as my Galaxy Nexus, it would be nice not to have to reach all the way to the top of the screen, as this requires me to adjust how I am holding the phone when I was previously in a "casual scrolling through items" position.
Ditto. However, it doesn't explain why it isn't on the iOS version as well.
The reason that I can think of is because Instagram refreshes (or rather pulls in) an entire new batch of photos. The expected UX for pull to refresh is that usually builds new data on top of the previous data, ie: Twitter, instead of loading entire new batch.