Again you are confused ... since you are using some of the $12K taxable income to make the ROTH contribution AND none of the $12K is taxable on your return then in your way of thinking you are not paying tax on the ROTH contribution ...BUT it is because none of your income is taxable due to your income being so low. So due to your low income ... none of your income is taxable AND post tax contributions (which is the definition of a ROTH contribution)... so in your way of thinking you are having your cake and eating it too.
