Asking defeats the purpose.

The point is that receipts make it extremely easy for IRS agents/tax officers to check whether a till actually registers all sales. They just need to make a few purchases and then later check whether what’s on their receipt matches the till’s memory. But it doesn’t work if they have to ask for a receipt. A portion of turnover will be declared anyway and it’s going to be that with receipts if those are optional.

Sure, a tax dodger might end up going through the garbage to remove the transactions corresponding to receipts that were thrown away from their records, but that's at least quite a bit of effort.