I believe most of those POS systems can operate in offline mode, in Europe at least. I have friends who work for large event organizers, and they have spoken about how if the system is offline the bars can continue to take payments, but there is a risk as a person's account may not have sufficient balance to make the charge when the system comes back online.
Most people here pay by card and I would say the vast majority use debit cards. A lot of people don't even have credit cards, unlike the US.
I'm no expert so may be wrong about some of this, and maybe huge events like these have these systems in place due to the risk of having to shut down bars etc. Many events are completely cash less these days.
