Why do they ask for your boarding pass in duty free?

Related questions

1. Does Heathrow Airport have showers?
2. Do planes fly over the North Pole?
3. When should I print my boarding pass?
4. Can you check in online for international flights?
5. How do I get from Heathrow Express to Terminal 5?
6. What do you do when you check in online at the airport?
7. Do you have to pay to drop off at Heathrow?
8. Can I print my boarding pass at the airport?
9. Can I print my boarding pass at home?
10. Do I need my e ticket at the airport?

Tags

1. #though
2. #which
3. #without
4. #claim
5. #retailers
6. #goods
7. #shops
8. #travelling
9. #known
10. #nothing
11. #proof
12. #other
13. #produce
14. #exemption
15. #there
16. #words
17. #outside
18. #course
19. #requires
20. #traveller
21. #cannot
22. #purchase
23. #boarding