I am creating this question in Meta to bring attention again to this question. I would like it to open reopened.
I am the OP, and initially I also felt it could a duplicate.
When I started thinking more about it, it became more obvious this has different shades.
It's quite obvious airports will make some pressure on airliners to allow things bought in the airport to travel for free. Airports are becoming more and more commercial centers, and shop rents are a relevant revenue.
Bags from duty free are quite obvious that was an airport purchase, but from other shops might not be. You can bring the same items from outside. I would like to know what's the Ryanair policy regarding this. Do they allow it, do they simply refuse or demand a proof that the items were bought on the airport?