"because if there are ways to save on taxes, or have your company pay you more for investing in a 401 k instead of putting your money in your piggy bank, and you don't do it, then you are turning away free money. you are saying 'no, even though you want me to earn more on my investments or help me not pay as much taxes, at no extra cost to me, I do not want that! I want to pay more taxes! and I want to earn less on my investments.'
and to do these things, is sin."
From the Holy Book of Bran 9:87 (Chapter 9, Verse 87)