Not Legal Advice: Each state can enact whatever laws it wants, subject to constitutional safeguards.
Federal laws are generally applicable in the same way across all state borders. However, under constitutional laws, states are allowed to create, implement, and enforce their own laws in additional to federal laws. This is because every U.S. state is also a sovereign entity in its own right and is granted the power to create laws and regulate them according to their needs.
Another reason behind this is that each state has unique characteristics in terms of factors such as:
Geography and natural resources
Location
Demographics of the population
Historical operations of business, commerce, and industry
Public policies and community standards in the state
Some laws, such as certain voting laws and criminal laws and statutes, tend to be somewhat uniform across states. However, some areas of law can be very, very different from one state to the next. Some types of laws that can vary widely across state regions include:
Gun control laws- these are often dependent on crime rates in the area
Child custody laws
Trucking and motor carrier laws
Business and corporate laws
Marriage licensing laws, especially with regards to same-sex marriage
Thus, one should be aware of legal issues that might arise when moving from one state to another, or when traveling to several different locations in the U.S. This especially true when it comes to licenses that are issued in one state, which my not be honored in another state. Read more at Legalmatch.com.
Browse laws by state and by topics at FindLaw.com.