Slixious: ... I suspect because they don't want to cover the liability of people spending money without PIN or signature out of your own account, rather than a credit card account.

No, when you tap your card, it usually goes through as a Credit Card transaction (either through Visa or Mastercard etc) even though the money will come out of your cheque or savings account. Therefore retailers may choose to decline this option to pay so they don't have to pay the credit card surcharges.

I will see say this much. I have seen the receipt show the declined transaction for tapping... out of a Savings account. Take that how you want.