Seems they book into economy in the US. These are the rules as quoted by Expertflyer for the ZDKBIZ fares:
CLASS OF SERVICE THESE FARES APPLY FOR BUSINESS CLASS SERVICE ON THE SK TRANSATLANTIC SECTOR.
----- THESE FARES APPLY FOR ECONOMY CLASS OF SERVICE FOR DOMESTIC TRAVEL WITHIN THE UNITED STATES TO/FROM/ VIA THE SK NORTH AMERICAN GATEWAY WHEN OPERATED BY UA/CO/AS/US
Z apparently not such a great deal unless you are flying to one of SK's destinations.