Mainly because all the US prices you see are excluding tax, so each state adds their own on (or not at all) at checkout.
In the UK we have one VAT rate for Apple computers, and it's displayed in the price (unless you're a business where VAT can be reclaimed). Once you add 20% to the US prices, convert $ to £, and add custom/import duties, you'll see it's hardly different.
Although for a while I used to buy my Macs from the German Apple store, as it worked out cheaper than buying from the UK Apple store. They also have different stock available in the Refurb store.