Price differential

I'm sure its been like it for ages but can anyone shed light on why Apple charge more in some countries than others. I've purchased items from the mac app store many time but I recently contemplated buying some mac software thats only available via the app store. The price in the US store is about $50 in the UK £49. Todays exchange rate for dollars into sterling is roughly 1.4 to the pound. At first I thought this discrepancy related to purchase tax but evidently thats not the case. Can anyone shed light on why apple pursue this blatant unfair policy.

Thanks