It's not. Since home prices are linked to interest rates (prices go up/down depending on interest rate movements), all that's going to happen is that home prices will go even higher. In the end your monthly payments is going to be the same.
The bank takes it's share in the form of fees. The negative rate is simply because the money market sees negative rates on the 5-10 year horizon. The bank simply passes the rate of the money market on to the consumer:
> Høegh said Jyske Bank is able to go into money markets and borrow from institutional investors at a negative rate, and is simply passing this on to its customers.
Negative bond rates happen when there is a surplus of capital looking for "safe havens". Some investors are willing to pay to have money placed safely for a number of years. The Danish real-estate bonds are regarded as a very safe haven.