Why does TurboTax add income from a non-income tax state to my state return?

Simply put, your state income tax is calculated according to the laws of the state in which you were a resident when you earned your income. 

Sure. We are commonly asked why income earned in states that don't assess income tax (Alaska, Florida, Nevada, South Dakota, Texas, Washington, and Wyoming) is included as income on a "taxable" state return.

For example, a full-year Oklahoma resident that earned income in both Texas and Oklahoma. She notices that TurboTax is including her Texas income on her Oklahoma return. Shouldn't the Texas income be excluded from her Oklahoma return because Texas doesn't collect income tax?

The answer is no. It is a widely-held belief that income earned in a non-income-tax-collecting state is exempt from taxation by the resident state. This is not true. In this case, our taxpayer's Texas income would be taxed as Oklahoma income because she is an Oklahoma resident and the Texas income was not already taxed.

Now let's say our Oklahoma resident moved to (or from) Texas during the tax year. Part of her yearly income came from Oklahoma sources and was earned during the time she was an Oklahoma resident. The remaining income came from Texas sources and was earned while she was a Texas resident. Can she exclude her untaxed Texas income from her Oklahoma return?

You bet. In this case, she would file a part-year resident Oklahoma return for the Oklahoma income she earned while she was a resident of that state. The income she earned as a resident of Texas would remain untaxed because she earned it in Texas while she was a resident of Texas.