I would be willing to argue that it is easier for some to believe that Christians might respond in such a manner because they have indeed seen them behave in such a manner in the past, in both minor and major instances.
A few years ago, this practice came to many people's attention here in Minnesota after an outbreak of tornadoes in the middle of an otherwise quiet summer day. Although tornado damage is always news, this received special attention from a well known local Baptist pastor as he speculated on his blog that it was a warning from God to a more liberal Lutheran denomination to repent of their stance on human sexuality. (That particular denomination was holding a convention and discussing human sexuality that day in one area that was affected by the storm.) Although his connections and conclusions may be intriguing to those who desperately seek such signs to validate their beliefs and made for some interesting responses online, I find them to be thin when taken in full biblical context and applied completely unequally from one natural disaster to another. There were multiple tornadoes in the state that day. Yet the only one that is seen as a direct warning from God is the one that occurred downtown. The ones that ripped through rural communities were not viewed as God's judgement upon specific farming practices. This theory also neglects the fact that "downtown tornado" caused the most damage in the southern area of the city rather than downtown itself. In fact, most Lutherans attending the convention had no idea that there was a tornado outside because they couldn't tell from inside the convention center. The convention did not have to be postponed. Meanwhile an independent record store suffered significant damage. Should we conclude that that was God's judgement on some of the music that was being sold there? The most substantial damage occurred in neighborhoods where massive trees were ripped down and uprooted. Does God have some sort of opposition to urban landscaping?
I'm sure you've heard of similar claims made nationally about major natural disasters. Many claimed that Hurricane Katrina was God's judgement on New Orleans' sin and involvement with voodoo/witchcraft. Haiti was the source of such claims after their massive earthquake, when some Evangelicals rewrote history and claimed the founding of the country was a pact with the devil and the reason God brought such calamity on them.
Minnesota is no stranger to tornadoes. We've had some larger and more significant ones since that time. So why do we assign meaning to some but not others? When a massive tornado destroyed portions of the small town of Wadena, there was no such similar claim. People were encouraged to pray and help. They did not call for Wadena to repent (although the town, like any town, must contain some sin). When I was growing up, I experienced an earthquake in western Minnesota. (It is rare, but it does happen.) It was a minor quake that struck during the middle of high school graduation, briefly interrupting the valedictorian's speech. We accepted it as a anomaly of the wondrous world that we live in. No one speculated that God was warning us about something or was upset with the young man at the top of his graduating class.
As I hear such claims being made, the only pattern I notice is that people only seem to receive such a clear insight on God's work when it is an issue to which they are strongly opposed. I notice that such claims are disproportionately made in regards to sexual issues and witchcraft. Few seem to ever claim that God is judging our greed, dishonesty, pride, or indifference to poverty. So how far do we take such claims? Do we have to look for messages in every weather pattern? Is God's will for my life determined by whether it is sunny, raining, storming, etc? I certainly hope not!
Whenever people are trying to figure out what sin caused a sickness or disaster, I think of John 9 where Jesus heals a man who was born blind. The disciples asked Jesus if the man was blind because of his sin or his parents. Jesus directly rebuked the idea that sin was to blame. He said that it was neither, but that it was so the glory of God could be displayed. Perhaps the message of God in tragedy is twofold. It reminds us first of all that life is fragile. It then demands, "How will you respond?" As we see the continuing coverage of Japan, will we look at it and try to pin blame on their prominent religious beliefs, cultural practices, or some dark spot in their national history? Or will we recognize our shared humanity? Will we mourn with them and assist them in whatever way we can? Is our job to remind them to repent? Or to reach out to them with compassion?