I'd expect QVC offers EZ Pay as the default because most likely more people use it than pay in full.
What retailer wouldn't want to be paid cash up front than carry no interest EZ Pay? I'm sure QVC would prefer payment in full at point of sale like other retailers, but obviously they found out they sell more if EZ Pay is offered.
A tip, try to remember to click on "payment in full", soon it will become a habit and you won't need to think about it anymore.