Crossing an American state line can change licensing, taxation, employment rules and much else. The variation is a structural feature of how authority is divided rather than an anomaly.
Powers are enumerated and reserved
The federal government exercises powers granted to it by the constitution, and powers not granted are reserved to the states or the people.
States consequently hold general authority over areas such as contract, property, family law, criminal law, professional licensing and most business regulation.
Each state has its own constitution, legislature and court system, so a state supreme court is the final authority on that state's own law.
Commerce is the main federal lever
Authority over commerce between states has been the principal basis for federal regulation reaching activity that would otherwise be a state matter.
The scope of that authority has been contested throughout American legal history, and its interpretation determines how far national rules extend into local activity.
Spending is a second lever, since federal funds can be offered on conditions, which encourages states to adopt standards the federal government cannot impose directly.
Pre-emption decides which rule governs
Where federal and state law conflict, federal law prevails. The difficult question is usually whether a conflict exists at all.
Some federal statutes state explicitly that they displace state regulation in a field, while others set a floor that states may exceed with stricter requirements.
Much litigation concerns which of those a statute intended, and the answer determines whether a state may regulate a subject at all.
Large states shape national practice
A manufacturer selling nationally often finds it cheaper to build one product meeting the strictest state requirement than to produce several variants.
That gives populous states influence beyond their borders, since their standards become the effective national specification through the supply chain rather than through law.
The same dynamic appears in disclosure and labelling, where a requirement in one state produces labels that appear on products sold everywhere.
Uniformity is achieved voluntarily
Where consistency is valuable, states adopt model codes drafted by bodies that exist for that purpose, in areas such as commercial transactions and building standards.
Adoption is a state decision and versions differ in detail, so nominal uniformity still requires checking the enacted text in each jurisdiction.
Interstate compacts serve a related function, creating agreements between states on matters such as licensing recognition, and some require federal consent to take effect.