Why would any major take flow through from their own regional?
Because if Alaska doesn't take their Horizon pilots, then they will go to AA/UAL/DAL/FedEx/UPS/SWA/HA, and the PAX that were flying on the Horizon plane feeding Alaska, will now be on a feeder for AA/UAL/DAL/SWA, because Horizon will have a grounded plane, for lack of pilots.