And for good measure...
I forgot to mention pricing issues, namely that the cost of living -- and thus what should be charged for a good -- will vary significantly from one nation to the next, and not just because of taxes.
E.g. due to the cost of living a book might cost the equivalent of $10 in the US, $5 in Nigeria and $15 in Japan. Or, demand might be very high for a book in one country (thus justifying a higher price, per basic macroeconomics) and low in another (thus justifying a lower price to boost sales, again per macro econ 101). So how much should the company charge, exactly?
Similarly, exchange rates could play havoc with pricing. One day a US ebook costs $15 to the Japanese customer, a few weeks later it's $12, six months later it's $20. Consumers are insanely sensitive to price fluctuations and won't be thrilled about situations like this.
So, unless the entire planet has the exact same cost of living and currency, setting a single international price is highly problematic.