To be fair to Tesco, the idea of the Price Check promotion was presumably to give its regular customers confidence that they could get their weekly shop for less at Tesco, by effectively promising to beat Asda's prices. We're guessing the theory was that shoppers might occasionally buy something for less in Asda - and thus Tesco would have to stump up the occasional 'double the difference' refund - but the net effect would be positive because it would ensure greater customer loyalty (not to mention hopefully pinching some customers from Asda).
However, Tesco clearly underestimated the Great British Shopper's eye for a good deal. Thanks to various money-saving websites, word got around that it was possible to target particular items that were on promotion at Asda, buy them on the cheap, and then by buying the same thing in Tesco stock up on double-the-difference coupons. Some went even further by using these coupons to buy more of the same items, and thus get even more coupons.
Now admittedly that's not quite in the spirit of the original promotion. So you can see why Tesco is a bit miffed. On the other hand, this 'cottage industry of savvy and determined people', as Tesco described them (and we don't think it was meant as a compliment), weren't doing anything they weren't completely entitled to do under the terms of the offer. You might even argue that they've just called Tesco's bluff - and in doing so shown that Tesco isn't as systematically cheap as it might like people to believe.
Either way, it's turned into a bit of a PR shambles for Tesco, which has now had to back-track twice - first by capping refunds at £20, and now by only paying out the difference, as opposed to double-the-difference. Cue lots of sneering from Asda about not making promises you can't keep, and so on. Damn those savvy shoppers, eh...