I just checked I-864. The statement is crystal clear:
You (Petitioner) is responsible for all your parents' expenses, including medical UNLESS:
1. Your parents worked and EARNED 40 credits
2. Your parents become US Citizen.
If your parent applied MediCare or SSI, you have to pay it back to the tax payers if got caught. If your parents apply US Citizenship, the application will be denied if USCIS found out your parents applied MediCare of SSI and USCIS will sue YOU for the money.