The duty free store is currently undergoing a major refurbishment.
The store owners and the airport company have a joint agreement where 'forcing passengers 'through the store and therefore encouraging them to buy products is beneficial to both parties.
I believe similar agreements are not unusual elsewhere in the UK,as the more the stores make in profits the more the airport take in rent or whatever the agreement is.Unfortunately not everyone likes or does the shopping side of airports but it is an important revenue maker for both parties and not something that is likely to go away soon.
What will be interesting is that if the UK votes to leave the EU,how is that going to affect duty free shopping in the future,as almost everyone will be able to buy liquor and tobacco at duty free prices,with the exception of those on domestic flights.Therefore returning to the good old days of pre 1999 when the EU abolition came into force.