How do large international sites like Amazon or TripAdvisor avoid cannibalizing with themselves?

If the content and layout of the pages on each different country domain are the same, how does Google differentiate these and know which page to serve? I've noticed that for some countries "Amazon.com" is served but for others the dedicated domain is? So searching for Kindle Fire from Spain will send you to Amazon.es but searching for Kindle Fire from Romania will send you to Amazon.com?