A more thorough explanation:

Mathews v. Diaz is a legal case that was decided by the United States Supreme Court in 1976. The case dealt with a provision of the Social Security Act that denied eligibility for Medicare part B to noncitizens who had not been in the United States for at least five years and had not been admitted for permanent residency. The Supreme Court held that this provision did not violate the noncitizens' due process rights.

The case was brought by a group of noncitizens who argued that the provision was discriminatory and violated their constitutional rights. The Supreme Court disagreed and upheld the provision, stating that Congress has the power to make rules governing noncitizens that would be unconstitutional if applied to citizens.

For example, a noncitizen who has been in the United States for less than five years and has not been admitted for permanent residency would not be eligible for Medicare part B under the provision. However, a citizen who has been in the United States for less than five years would still be eligible for Medicare part B. This distinction between citizens and noncitizens is allowed under the Constitution.

The Supreme Court also stated that immigration policy requires a distinction between citizens and noncitizens, and that Congress is free to extend benefits to any sub-group of noncitizens it finds reasonable. The durational requirement and admission for permanent residence are simply markers Congress has chosen to delineate between noncitizens who have developed an affinity to the United States sufficient enough to warrant receiving certain benefits, and those who have not.