Let's say an employee earns $500 before tax and has to pay 20% income tax. The employee will take home $400 and $100 will go to the taxman. If the employer paid that $100 income tax instead of the employee and paid the employee $400 it would make no difference to the employer, the employee or the taxman. Income tax is therefore an indirect business tax on human productivity.
So, why do employers pass this tax burden to employees? The answer to that is simple. When businesses replace human labour with automation, they no longer have to pay that tax either indirect or directly and therefore the government lose out of the tax revenue and society is poorer because of it.