Well, it's a conjecture. The stock price is sky-high and this should mean that investors expect Amazon to grow much, much larger. International expansion seems to be the best way to go about that.

For the long-term survival of Amazon it might also be a good idea to try to dominate everywhere. In every market that Amazon doesn't serve, a competitor will take its place. There's a danger that in the long run such a competitor will become large and efficient enough to compete with Amazon in Japan, UK, Europe, or even the US.