"But officer... I was just thinking 'I wonder if I can mug this person and bludgeon him to death' so I did it, and to my amazement it worked! Honest, guv..."
Just because you can do something, doesn't mean it is legal or morally acceptable. You know that you didn't acquire the vouchers genuinely, and it wasn't an innocent mistake, so your entire argument is defunct. As a hypothetical situation, granted, someone might be disappointed if they – at a million-to-one chance – discovered the loophole by accident. That's why Sainsbury's are giving those they have contacted the option of coughing up the proper amount or cancelling the order. Like it or lump it, Sainsbury's have done nothing wrong here other than failing to cater for malicious users exploiting their generosity by breaking their T&Cs. I know what my stance on honouring the discounts would be if I were in charge at Sainsbury's...
*av