Country restrictions on apps?

I've noticed and heard that there are some apps that aren't available in some iTunes app stores. I don't know why this is, but I can guess that with certain apps, Amazon's Kindle for example, there may be legal reasons why an app can't be released in a specific country.

However, I've no idea why that might apply for music apps. So why do some them not appear on all stores? Any devs out there can give a comment?