In truth they ask to see your boarding passes I think to reassure themselves that you are actually travelling - like the two in a row security checks plus passport control aren't enough. Once they establish you didn't come in through a skylight, they couldn't care less how many you buy, it's not their problem. I saw exactly the same in Prague, people on our flight not allowed to buy duty free fags but buying loads - each one had to produce their boarding pass, which was checked and on all occasions they bought all the fags they had in their basket.
Sadly on arrival at Edinburgh, not a single official in the customs area at all.