In a lot of places these days, you can't even buy something with cash when the register is down. It's part and parcel of the accounting system, the inventory system, and the payroll system (who's at what register and when). And where would they put the money, and how would they make change (even if they know how). So it's a complete shutdown.
I saw this first hand in Santa Cruz in 2009 after fiber optic lines from San Jose were cut in an act of sabotage (probably Union).
We have bought things here when the system was down, this is not the big city.