Unfunded pensions are NOT confined to USA. UK civil servants also receive pensions which are not fully funded. Consider: I am a civil servant who retired at 60 after 11 years' service. I receive a small index linked pension of £3500 pa. Should I live to, eg, 90 - why shouldn't I? I'm fit and healthy - I will receive a total (at today's value which index-linking is) of £105k. I have contributed approx £11k over my working service. Therefore, after three years, I will have received ALL my contributions back. For the NEXT 27 years, I will be living, in effect, off the taxpayer.
It was for that reason (amongst others) that, when my (former) union went on strike re the government changing the rules, I refused to strike. I would have been ashamed. Some of my colleagues thought the same and worked through the strikes.