Why does non-taxable income from Alaska, Florida, New Hampshire, Nevada, South Dakota, Tennessee, Texas, Washington, or Wyoming get taxed on my resident state return?

If your resident state collects income tax, you’re still required to pay taxes to your resident state on any income that was earned in a "tax-free" state. You cannot avoid state income taxes simply by working in a tax-free state. You have to also be a resident of that state.

Similarly, if you are a resident of a tax-free state, and have worked in a taxing state, you still have to pay taxes to the state where you worked. Unless you are working in a reciprocal state, you will have to pay taxes to the state where you earned your income. You would file a nonresident return to pay these taxes.