I don't know if you can get too mad with John Lewis not taking Amex; you want to use Amex for the great kick backs they give - but they pass the charges on to the store.
Stores have to pay an extra couple of % to Amex than they do to Visa and Mastercard; precise sales figures for John Lewis aren't available as they're collectively owned, but I wonder if taking 2.5% of the sales off a store's profits would mean they were trading at a loss?