When a company buys another company, they don't necessarily merge them. Sometimes they are kept as separate entities, as I believe ANZ & Ansett were. Synergies are sort for cost savings & the profits flow to the company that did the acquiring (the owners). But all the income, expenses & liabilities are quarantined in the individual company.
ANZ may have had a moral obligation to look after Ansett & its staff when things went bad (people will have different opinions on this), but they had no legal obligation to do so. And in order to save themselves, they wrote off the acquisition & left the company to fend for itself. Sort of like ignoring pleas to purchase more shares in a defunct company (throwing good money after bad) & dumping the currently owned, useless share certificates in the trash.