The Data Pack has put together an unofficial and incomplete list of banned hashtag searches, and some of the results might surprise you. The majority of the tags are your run-of-the-mill NSFW pornography terms, and others, like #thinspo, are blocked to protect individual users. As for those pretty generic terms like #iphonography, well, those are simply too broad a search term. "We’ve stopped serving feeds (both in app and API) for some tags that were too generic and didn’t provide enough end-user value," Instagram's Mike Krieger noted in a Google group.
The problem here, though, isn't that tags like #license aren't banned (although they probably should be), and #IG is. It's the very act of users uploading information that puts them at risk of identity theft. Perhaps issuing something like an Instagram General's Warning might do the trick. But, at the end of the day, it's simply about increasing awareness, and putting faith in user discretion. (The Data Pack)