From my understanding of taxes, you can only be taxed on money earned received as a present... However if let's say you made $100,000 during the year and paid tax and are now left with (for example) $60,000 and spent $1,000 and the store owner decided to give you back some of that money (let's say $100) you don't have to pay tax on that$100 cause you already paid it together with your original $100,000 so in other words cash back isn't taxable, however a signup bonus is a present to you and was never taxed to begin with therefore they tax you for it, if anyone has data points against this please let me know, thanks