Sign in Bristol Duty Free says ALL airlines serving BRS allow a duty-free bag in addition to their normal allowance. This would include FR.
However to apply different conditions to the return journey than the outbound sounds to me to be a breach of their contract with you.
Probably wouldn't do much good for you but you might want to pass your experience on the Norwegian NEB and/or your Consumer Affairs people. They might be able to put some pressure on FR, who would ignore you. The NEB is:
Luftfartstilsynet
Postboks 243
8001 BODØ
Norway
[email protected]