Putting in your cart is not the same as buying it because you haven't made any commitment. If you haven't made a commitment, why should they?
Because it is identical to walking around a B&M, putting something in your cart with the shown price and then when you get to the register, sorry price went up. I worry about this case with stores using electronic displays for price. They could pull off the same BS. Otherwise what would prevent them from always raising the price the moment you put it in the cart and claiming it was a 'normal' price increase?