Because steam's products are digitally distributed, and their prices were exclusively US dollars before. Not only that, but the british prices scale somewhat equally with the north american prices, while the rest of europe gets shafted, countries that haven't even adopted the euro are forced to use it.
This is just blatant thievery, nothing else. But I won't write and complain, create boycott groups or anything else, I just won't buy games at their prices, and if needed be, I will have a friend abroad buy me a particular game as a gift, rather than this ridiculous scheme.