Having an Eticket issued to a final country does not necessarily mean anything more than sometimes (often) it is cheaper to book a ticket to point D via point B&C than to terminate the journey in B or C.
Canny travel agents and passengers do this all the time.
It is called cross border selling. The pax has no use for the final coupon(s) and they just nosho for it. Nothing the airline can do about it, and pax has got himself cheaper fare.
If you have the correct ID you can disembark even at transit points, and enter that country with practically no questions asked, expecially within the EU. Arriving pax with an EU passport are not cross checked with the flight manifest or whether their ticket actually permits them to stopover or only transit point B or C from an airfare point of view.
Immig is not interested in cross border selling of tickets. They are only interested in whether an arriving pax has the correct documents to enter their country.
The US in this regard is much tougher- hence queues to enter the US take several hours to get through.
An EU passport holder arriving within the EU, has their passport scanned and off they go...if that.