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...
When they were legally forced to do so, the government prevented them from being sued to do it anyway, by destaffing the only judicial office where such procedures could be started.
Also, banko Santander was sold to external investors, with direct involvement from the central bank and the Spanish government, and ... failed to mention these negative-interest-rate obligations. If anyone else ever did anything remotely approaching this they wouldn't just be nailed to the cross, people would be talking about it for decades too.
Oh and despite all this cheating, banks and governments not respecting their own laws (nor even taking the time to change them, or even so little as involve the legislative branch in the process), despite violating every precept of democracy on both the Spanish and European level ... all large Spanish banks are a hair away from bankruptcy.
https://www.nytimes.com/2016/12/21/business/spain-banks-mort...