While "amok" is typically described as an Indonesian word, it is actually a word of Malay origin. The traditional Malay language has been the dominant lingua franca in the region that these days contains the countries of Indonesia, Malaysia, and Singapore. As a result the modern languages spoken in these countries (Bahasa Indonesia and Bahasa Malayu) are heavily based on this old language. Thus the word "amok" is present in both of them.

Personally I suspect that the word is so commonly associated with Indonesia because in this country the dominant island (politically and economically) is Java (that's where the capital Jakarta is). Java just happens to be the most densely populated island in the world, so they have more then their fair share of people running amok there.