What happens when you don't have enough money available to Amazon to cover all the things you grabbed? Do they give you a window to return items before banning you from the store and/or notifying police?
I'm assuming your Amazon Go account is tied to a credit card. In which case, it's not Amazon's problem. They got their money - you now owe your credit card company.
And if your credit card company declines the charges when Amazon attempts to bill them? Then they're out inventory and don't have any compensation. Then it becomes their problem.
Maybe they ping the card with a $1 charge every so often to ensure it's at least not cancelled. It could be maxed out in which case they'll likely just give you a negative balance and a time period to pay it. If you don't they can just sell it collections and potentially ban you from the store (it looks like you need to scan your phone to get into the store).
They could do like gas stations and charge a hold just whenever you enter the store then finalize the transaction whenever you leave. That'd solve the problem and if it was going to fail you'd just never be able to enter the store and you'd just see a warning message about your CC not being valid.