Legs tearing themselves off is not horrific to starfish. The horrific bit is the wasting part of the disease, that they start to rot while alive.
Having their legs torn off with part of the center is quite a normal occurrence, it is a survival mechanism and a frequent event in starfish life. They can regrow legs and regrow from a leg that has part of the center attached. In fact if they didn't have wasting disease, the original starfish would regrow its leg and the leg would regrow a new starfish, so there would be two of them.
Most likely the legs detaching themselves from the sick host is a survival mechanism, as only the healthier legs are doing it. Problem is once they've gotten whatever it is they've gotten their normal supernatural regenerative powers are no longer working, so both parts die.
The fact that all the news stories about this focus on the leg detachment shows very little understanding of starfish. That being said the huge numbers of die offs and rotting while alive are indeed horrific. So this is a horrific event, just not for the reasons that have made every headline.