It is no crime to get airside without flying.
When I'm on a business trip it happens quite often that I have to return to my office and either take a later flight or cancel the ticket. Where should there be a crime?
I often check in for all the later flights on a particular day to be sure I make it on a plane... Is that a crime as well???
It would be a crime to get airside, buy duty free, return ourside without boarding the a/c and requesting a refund. But there the crime does not concern the getting airside and return but the tax issue when you buy duty free without leaving the country (or the trade area).
You only run into the risk that airline staff will recognise you and deny you the vouchers or bring you into paperwork for the refund....