This means banks need to pay the SNB on their funds deposited there. Many of them pass that on to their customers: Deposits don't earn any interest to speak of, and for large sums (starting from 1M or so), banks commonly charge interest. At some points, the yield on Swiss Gov't bonds was even negative.
If you can offer a safe (and bank-accepted) way to store that money, you stand to make a killing.
On the other hand, even in that environment, mortgages aren't free. Rates range from 0.5 to more than 2%. Part of that is the banks' margin, part of it comes from fixing the interest rate for some (2-10) years.
[1] https://www.snb.ch/en/iabout/stat/statpub/zidea/id/current_i...