To help see the intent why NY state tax calculation is done that way,
Hypothetically, Consider a person earns $5,000 in each of the 50 states. His annual wages would be $250,000. If each state just considers the income the person made in the state ($5,000), he might end up not paying any state tax.
If you over simplify, states are trying to calculate the tax bracket considering your overall annual wages ($250,000) and apply that bracket to the income made in the state.
