From this link....
A college education for illegal aliens
...In September 1996, in the landmark Illegal Immigration Reform and Immigrant Responsibility Act (IIRIRA), Congress prohibited states from giving in-state tuition rates to illegal aliens. Members of Congress evidently never imagined that some states might simply disobey federal law.
I believe, and someone correct me if I am wrong, that states are not prohibited from giving illegals instate tuition, but the law is IF they DO give it to illegals, they are then forbidden from charging out of state rates and must charge all US Citizens, regardless of residence the same tuition, ie in-state tuition.