Earlier quoted context omitted.
I agree, but almost everyone today can use a computer or smartphone. They can type in their income, and the computer can calculate it, providing them an average number of what their percentage of actual taxable income was- I think Turbo Tax and other software might do something like this. They don't have to understand how it works to do their own taxes.
They can, but what would make them think their initial misunderstanding is even wrong? While some might think "surely the system isn't like that..." many would default to "of course they want to steal my money" etc. Not to mention the amount of misinformation that can get passed around
If they are curious, they can type in three different incomes, even if they aren't planning on submitting the tax form with the wrong amounts. So if their income is $59,000 and their tax bracket is say, 15% for that, instead of say, 24% for 60,000-the next end of that bracket- e.g. $75k or $95k) they might get a much smaller tax return if they type $60,000. But then if they earned a little more, like $61,000, they could type in that and see what their return is, along with the rate.
If it is a gradient with smooth progression for each dollar increase, then they can immediately see how much more they are paying in taxes from 15% to 20%. But in reality, in a smooth/gradient-based progressive tax increase, those amounts would be centered in the middle of those brackets (like 15% would right in the the center between $35,000 and $59,999, or $48,500, and the $59,000 income would be closer to 19.9% and the $61,000 income might be $20.1%.
It wouldn't be considered fraud if they are just determining what they would pay if they earned a little bit more. In fact, if they just discovered an invoice check they received from self-employment, and forgot to report it, they might have to type in different amounts, if they didn't report the right amount the first time. Everyone would pay the same tax rates at each income level (with $1000 intervals, or even $1 intervals.
The system could still be transparent because it can show a chart of the tax rates for each $1,000 increase, even if the tax brackets only provides an estimate for each bracket. I do agree some people can't read charts very well, but if they included 100 thin bars for taxable rates under $100,000, people would be able to see the estimated percentage that they would be paying. It might be 15%, 15.5%, 16%, 16.5%, or 17%, and so on for $48,500. $49,000, $49500, $50,000, and so on, respectively.
The idea with this is that it would disincentivize people from misreporting because if they report earning $500 less, they aren't saving much more money (not to mention illegally)- their tax rate might be 0.1% less, and on a $49500 income that might only be $250 less in taxes over reporting $50000. The risk of getting caught is not to going worth it if this method is in place. (Actually not in any case, but at least they can see it's not going to be a huge decrease in taxable income by reporting a little less, so the only way they can attempt to pay less is by reporting a lot less, and the penalties for that would be larger)