The payroll tax is not officially a tax since it is officially considered a premium towards social security, which is officially not a government program but is a type of insurance annuity (okay, if it was a real insurance program, it would be closed down immediately as a ponzi scheme, but that’s a different matter). Even though the tax is highly regressive, the Congress didn’t object since after all, it isn’t a “tax”.
Remember, being a politician is to learn how to put your hands in someones pocket without calling it theft. They’re really quite clever at it.