They kill them because they cannot control the spread of the flu/virus. If you kept the ones that survived they could actually carry the virus. If you got new stock it could get them sick. It also can be mandated by the authorities I recommend doing some research online about how it is mandated.

Rocky coasts (high and low relief) result from resistant geology (to the erosive forces of sea, rain and wind), often in a high-energy environment, whereas coastal plain landscapes (sandy and estuarine coasts) are found near areas of low relief and result from supply of sediment from direct terrestrial and offshore.