Well, first off, we need to clarify what is typically meant by the terms medium and psychic. A medium is most often thought of as a person who communicates with people who have died or or not present in this physical world as we know it. A psychic seems to encompass a whole lot of things from being able to predict future events (Precongnition), look into "past" events (Retrocognition) such as who murdered Jimmy Hoffa to being able to do "healing" work.
So when you say John Edward should do some good by preventing a disaster, that is not able to be accomplished as far as his mediumistic skills go. I have read he has predicted things before but I don't know if any were on a "disaster" type scale.
I realize that many people are wary of psychics and don't believe there are ANY psychics. This is simply the way you choose to view it and you have the right, of course. But...here is where the problem arises. In the true essence of research into whether or not they are real, accurate, etc., is to visit a few and THEN judge for yourself, but that is not going to ever happen because you REFUSE to go to one because you don't believe in them.
If you are only looking to psychics on television and in books, then again, there is where the problem arises. (Not to say they aren't real, just that those are the ones most often ridiculed)If you want to truly find out if psychics and their abilities are real, then you should seek out those who live in small rural towns who sell nothing, write nothing and have no vested interest in anything except to help those who have come to them as a means of providing guidance. These are the best psychics around. You may ask, "why then don't we hear about them on tv if they are so good? Huh?" Well, that is the whole point, they DON'T WANT the fame and fortune so you never will hear about them. THESE types of psychics are akin to getting guidance from your pastor, your therapist, your best friend, etc. Many people discredit them for being vague and given generalizations but there are many who do nothing of the sort. "Why should I take YOUR word?", you ask. Because you aren't willing to find out for yourself, so taking my word is all you got and I don't want people to take my word. I want people to judge for themselves but most don't want to throw away money on fakes and charlatans and I certainly can understand that. I am in no way defending Sylvia and John Edward, it is just to give you some perspective to try to answer your question as to why they don't stop disasters.
Let me ask you this...If you were getting ready for bed tonight and you are stopped in your tracks frozen and see the bedroom in front of you disappear to be replaced with a vision of a major disaster (lets say a 10.0 Earthquake in Chicago)and a voice tells you that you can save many lives, what would YOU do? Tell someone? who you gonna tell? the news? who do you think is going to listen to you...you are not a psychic and you don't believe in that stuff. So, you tell no one because it's not your concern, right? After all, you didn't ask for that vision. This might give you an idea of just how much responsibility it places on someone who can see/is shown them. Just curious...