Trading security for convenience
Making something convenient usually means giving up some security. In this case, we've traded the any form of authentication for the convenience of paying up to £30 10 seconds quicker.
I recall issues where M&S were reading chips/cards of nearby people, rather than the actual customer and there was the POC where someone wandered around a railway station with a bag containing a battery powered card reader and harvested hundreds of pounds in minutes.
This is a dumb concept, on the lines of the automatic door release on cars if an impact was sensed on the bumper. Kick the bumper and you can burgle the car in less than 10 seconds.