"When you can borrow at a rate below inflation, you're borrowing for free," McBride said. "This is the concept of using other people's money and it preserves financial flexibility for the borrower."

If you're borrowing at under market rates, we, the market are effectively subsidizing you. Does Zuckerberg need subsidies in order to have a mortgage?

The mortgage was likely only one piece of the deal negotiated. It costs the bank nothing to write an at-cost mortgage, but gives Zuckerberg value though additional liquidity. Items that cost nothing yet provide value to the other side are excellent things to throw on the table during negotiations to help seal the deal. One example: The bank may have the adjustable rate tied to the balance in an asset management account -- if Zuckerberg takes his money elsewhere, the mortgage also becomes profitable.