The Constitution is a document that is specific in some instance such as the three basic qualifications for the presidency yet vague on other issues such as the regulation of commerce. So when people talk about this or that being constitutional, they are really talking about how the courts interpret the Constitution.
Most Americans are probably not aware, even though it is taught as early as high school, that the reason for our Constitution were the disputes between states over commerce. There was intense rivalry between the states under our first set of national laws, the Articles of Confederation. States passed laws regulating business transactions with neighboring states to gain an advantage. This led to retaliation by neighboring states. Each state also had its own coins and currency which further hampered business transactions across state lines.
Each state was sovereign and carried out its own individual commerce or business with foreign nations also. Dealing with 13 different states with 13 different sets of laws did not promote good international trade. The competition between states in foreign and domestic markets hurt all of them.
Alexander Hamilton, a businessman, was the person who pushed for a revision of the Articles of Confederation to address these problems and strengthen the national government. Rather than a revision of the Articles, the state delegates created a new set of laws that completely changed the relationship between states and central government by granting more power, with limitations, to the national government.
Limitations were necessary because the states had finished fighting the British just a few years before and there was a genuine fear of a central government with too much power. So the Constitution specifically stated in Amendment X that, “The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.” Time, circumstance and court decisions would change the meaning of this amendment.