The norm is for airlines to cancel all remaining segments if not flown in order. There was however a court decision in Germany banning that practice so for ticket from that country, airlines would be able to reprice the ticket and take an additional charge. I suspect the ticket you are looking at may be from Germany. As it happens, with last minute repricing based on the itinerary actually desired plus a penalty, it may not be any cheaper than just buying something from scratch with another airline so may well be a Pyrrhic victory for the passenger.