The United States constitution has protection written into it for individual states so that states retain certain individual rights. The tenth amendment to the constitution states that the power that is not delegated to The United States by the constitution nor prohibited by it will got to the states or the people.
A good modern day example of states exercising their individual rights pertains to the legalization of marijuana. Several states have legalized marijuana for both medical and recreational use while even more states have legalized marijuana for medical use only. In contrast there are still some states that do not have legal marijuana at all.