[identity profile] nicegeek.livejournal.com 2012-04-24 01:17 am (UTC)(link)
The US also has income tax deducted by the employer. However, most people have some other form of income, be it bank interests, investment dividends, or a second job, and there are also countless potential deductions and credits buried in our insanely complex tax code. Also, since the tax rate is graduated, the correct tax rate can only be fully calculated after a person's total income for the year is known. Consequently, the pre-paycheck deduction is only ever a rough approximation, which everyone needs to rebalance every April.

Hence the appeal of the EIC/flat approach.