What merit good does the u. s. government pay for through the medicaid payroll tax?

The answer is Health care
medicaid payroll tax is one of Government's way in conducting wealth distribution for the citizens.
Through the payroll taxation system, the government aimed to take income tax from higher earning individuals and distribute lower cost/ free health care to the people in need.

the medicaid payroll tax pay for health care for the elderly that cant afford health insurance. It is in addition to their medicare health insurance.