We have a Tesco loyalty card and the points are really quite valuable by the end of the year when we buy the kids Chrismas presents, they also off money against holidays etc.
I can imaging a rougue till operator swiping their own card could be very well off at the end of it (buying TV's etc on offer with the "points").
The reason they may have asked is if you really did have your own loyalty card and they bagged your points they would have been caught red handed!.
The loyalty card is not linked to any debit or credit card, we use two different cards, and cash using the same loyalty card with no issues.
To me it sound like the till operator is on a good little earner
.