It isn't according to the law!
They are asking you to pay income tax and they don't care what gave you this income, they just want the money. That's why the law states what it states.
According to the tax law, that is basically the same for every country, even if you get cash for mowing your neighbor's lawn, or find money lying on the side of the road, you are supposed to add it to your yearly income and deduct the tax. Nobody does that, but that's what they'd like you to do.