How is it not illegal and more importantly ethical for Additional Tier-1 bonds to be written off in case of rescue or liquidation of the company?

Additional Tier-1 (AT-1) bonds issued in India, as I understand is regulated by RBI, the central bank in the country. It seems if the company faces severe losses,the RBI can then activate a Point of Non-Viability Trigger (PONV) and write down the AT-1 bonds. How is it legal and ethical to do so?