The government takes their taxes, one way or another. Traditional 401(k) accounts allow you to contribute using pre-tax dollars, meaning when you start getting 401(k) disbursements at retirement, that is considered taxable income and they tax the disbursement.
Roth 401(k) accounts allow you to contribute using post-tax dollars, so the idea is that when you get retirement disbursements, you have already paid taxes on that money and it shouldn't be taxed again.
I don't know how this squares up if you have multiple income sources at retirement, such as a Roth 401(k)+Social Security+Pension+Any other retirement account, that total income would still require some kind of taxes paid, even if it's "not taxing" the Roth disbursements themselves. That is one argument people make for Roth stuff though, if you think your retirement tax bracket is going to be higher than your working years tax bracket, you'll "get more" out of your Roth 401(k) than a traditional 401(k) since you paid in at X tax bracket but disbursed under Y tax bracket.
Someone who knows better than I do, feel free to correct me.