US courts do not define new legislation. They only interpret existing laws (there is gray area in how the interpretation impacts those how those laws are upheld

US courts do not define new legislation. They only interpret existing laws (there is gray area in how the interpretation impacts those how those laws are upheld). Other rulings are often referenced in a decision, but it is possible to have different courts give conflicting rulings (see, for example, http://www.employerlawreport.com/2009/05/articles/labor-relations/two-conflicting-federal-circuit-court-decisions-issued-today-call-into-question-all-nlrb-opinions-issued-in-the-past-year/ http://www.employerlawreport.com/2009/05/articles/labor-rela...). In that case, it is often up to the Supreme Court to decide (but, as far as I know, that is not required) and, if the legislature doesn't like that interpretation, to modify the laws appropriately.