It's not unheard of in Europe to get 2-3% discounts if paying by cash (particularly in computer stores). The reason is that in some countries, merchants pay that percentage of Visa/MC commission (and in case of Amex even more) plus a fixed fee per transaction. For that same reason bars usually accept cards only over a threshold of 5-10€ or so. Though if that happens in hotels indeed it is strange then.
