2 things they don't have to make themselves or ship in, save them money, less weight for shipping, saves them money, still charging the customer the same amount.
Do some quick math, you say £5 to the customer, but add all of the backgrounds costs up, so lets not pretend that its all about the environment eh, Apple are a business, a business that has shareholders and CFO's to appease, the whole green thing means people can charge more for less under the guise of it being environmentally good when, in reality that's not their primary driver...