I guess what I don't get is that there is a clear pietistic Islamic tradition of depicting the Prophet, e.g.:
http://www.faithstreet.com/onfaith/2010/05/06/why-islam-does-not-ban-images-of-the-prophet/2383
So from an outsider's perspective, it comes off as a bit of historical revisionism and cultural conditioning (if not cultural amnesia). Culture changed that sought to outlaw what was condoned and even encouraged in Islam's past. And followers have been conditioned to feel insult ever since.
Just as when Catholic conservatives like the claim that Catholic priests have always been celibate, that was not the case until tithing and land-ownership became an economic issue for the Church. But people just recite it as if it were the absolute truth, not even examining the factual history of their own faith.