As Jon slept, the stone began to work on his wish. While this wish would normally not have been much trouble, due to the stone's range limit, there were certain forces at work that would ensure that the wish would cause as much trouble as possible.
Unkown to Jon, and most of the world's population, certain stars had alligned in a manner that caused the power of the stone to amplify. This caused it's range to expand from a few miles, to near infinite.
In other words, Jon's wish could not only have an impact on his town, but the entire planet, or another planet entirely.