The underlying principle within the Constitution is the sovereignty of the states in all domestic matters. The Federal government has no jurisdiction in a State of the Union and the citizens of that State. It cannot assess or collect taxes where it has no jurisdiction. It does have, however, direct taxing authority in those territories that it has purchased - District of Columbia, Puerto Rico, Guam, the Virgin islands, American Samoa - and any foreign business or corporation operating within the geographical United States. These kind of taxes are called "excise" taxes. They are taxes on a privilege. The government is granting foreigners the privilege to operate a business within the states.
Believe it or not, the government does not have the Constitutional authority to directly tax any American business owned by an American citizen. It has no authority to directly tax the wages of an American citizen. For the American citizen, operating a business and making a living is a RIGHT, not a privilege. A right cannot be taxed. Only the State has the authority to tax its citizens if it so chooses, and that in proportion to its population. The government has the authority to tax any foreigner working within any State, but not a citizen of that State. So how do you explain the 16th Amendment?