It is the way codeshares work. They are not intended/allowed to be sold for local traffic, but intended where the longhaul flight is with one airline and connecting to shorthaul flight with another (eg if I fly SIN-SYD on SQ I can then use SQ code for SYD-AKL but I can't use the SQ code SYD-AKL by itself), OR as in the above example to serve a particular market (ie either destination or origin is home base to the airline but it doesn't operate that route).