BAA wanted to keep the price of items the same, to make things simple. I'm not sure if retailers have to go along with this or they're just encouraged to do so but they all seem to follow it.

Exceptions for cigarettes and alcohol where the duty can be quite high. Although sometimes alcohol has the same price both EU and non-EU.

Other duty free shops around the EU seem to do the same, it's rare I see ones with separate non-EU pricing.