This minimum card payment is not uncommon in Switzerland at mom-and-pop shops, in my experience. Chain stores and larger businesses always accept card payments, but I still need cash once in a while. I haven't actually seen this in restaurants in the US in a long time, although I have at some other small businesses, so maybe it's also somewhat regional.
Payment networks are relatively expensive in Switzerland and some contracts still charge a flat fee + percentage on every transaction, that's why you see these minimums at bakery shops and the like. Hopefully they'll eventually go away.
