Good for you OP but to be honest, I think Sainsburys grossly exaggerated here. How old are you? If you were born 98, you frickin 17 years old. I would have taken these coupons, too if I were that young. I mean, they are not that much worth anyway, a few pence or 1-2 quid per receipt max.
Honestly, is this a ''police state'' we are in? All they could have done is, talk to you about it, see your reaction and then tell you that it was ''wrong''. You are a human being! Not needlessly scare you, holding a meeting which takes a week to decide the ''verdict'', and threatening of potentially sacking you. As if Sainsburys didnt make millions of profit anyway. You probably made 5 quid profit. They are the ones who look ridiculous.