Duty Free is allowed - the airports all complained. Many airports give Ryanair discounted landing fees etc as they would make money from things such as duty free sales. It was (for a brief period) stated that all duty free must fit inside your cabin baggage, this rule has now been reversed.
All airports should be following the same rules - but this is what happens when you have so many different companies being contracted out to deliver the same product.
EDI for eg is very strict with the policy - I have seen PAX offloaded from there when travelling through. However, AGP let pax through with handbags and trolley bags.