Actually the employer is only allowed to withhold taxes because the employee agrees to have taxes withheld 'as if they were due.' Then when the employee files a tax return, he swears that he owes the tax, and the IRS happily accepts his perjury. If the employee refuses to agree to have taxes withheld, the employer has no right, or duty, to withhold. This fuzzes off the employer something fierce, since he stupidly believes he has a 'duty' to collect the tax, although the W-4 clearly protects him against liability for any uncollected tax.
I am inclined to believe that the income tax acts as an excise tax on the privilege of using Federal Reserve Notes (debt obligations), thus it is uniform over all forms of income. How the IRS justifies different brackets is still a mystery.