There are some down sides though in that now banks know exactly what you are spending your money on and stores are now facing increasing terminal fees from these transactions ( in Australia anyway).
Just to be clear - we talk about 2nd biggest city in Switzerland and life in and around it.
Tax evasion, avoiding deal with chargebacks, and not having to pay merchant account fees. However, those fees are nearly zero for debit card accounts, and chargebacks are pretty much non existent with Chip + Pin (or Apple Pay or EMV), so I would most likely guess tax evasion. I've been to many restaurants that offer 10% discounts for cash payments, versus card, which works perfectly as they get a portion of the sale on the record for plausible deniability, and a portion off the record.
Granted: germany and france are a bit behind in this.
This country isn't what it used to be.