Android Design guidelines actively discurage this pattern - it supposedly too "iOSish" and protected by patterns.
http://android.cyrilmottier.com/?p=598 is a really nice blog about why it's an anti pattern on android (and no, it's not to be different from iOS).
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.