I'm having no trouble imagining all of the wonderful reasons the government might have for preventing Americans and their money from escaping it's borders. And all of the things that come after that. I suppose I can think of worse, but I'm afraid because we already know what happens when countries slam the door on their citizens and allow only the annointed to travel. It's never ever anything but reactor-grade awful.
If the government does things you don't like and you are allowed to leave then you can do so and it ends up being not that bad. I would say that not being allowed to leave is practically a prerequisite for having something to fear from your government.
Or did you mean the scarier things are like the Yellowstone Caldera erupting or Godzilla attacking?