The principle would be the point, Apple. So that people who are getting something, (in this case, an income) from the government, can have "skin in the game" as Joe Biden likes to put it. Your own brilliantly articulated point, that all income should be taxed the same, also comes into play here. Why should someone collecting $20,000 not have to pay taxes? If Donald Trump made $20,000, you'd fully expect him to pay tax on it, wouldn't you? If Mitt Romney only made $20k in stock trading last year, you'd still expect him to pay taxes, wouldn't you?
So in essence, what you are trying to say is, all income should be taxed the same, except for government income, which shouldn't be taxed at all. The only nagging problem with that philosophy is, people would tend to want tax-free government income as opposed to regular income. Especially considering, there is no labor or work associated with the tax-free government income, no clock to punch or job to show up for, no boss to answer to, no tax forms to have to fill out, or receipts to have to keep track of.... Yeah, sounds like a pretty sweet gig to me!