Before blaming Ryanair too much for allowing them to link remote airports to major cities - it's worth noting that from a scheduling standpoint it is IATA which controls the airport-city link so it is only on their approval that Hahn was linked to Frankfurt. Perhaps someone from LH could verify this - but my understanding is that LH Cargo wanted to develo Hahn as a cargo centre - lorries connecting to aircraft etc - and wanted Hahn designated as an FRA airport for selling purposes. On this basis I understand LH went to IATA and with the agreement of FraPort who own FRA Main it was agreed.
I guess it's not the case with all Ryanair secondary airports, but in some cases they were not be the initiators of the designation.