Amazon usually bill in arrears (no idea about Lightsail). In the general case even, it doesn't matter. What you owe Amazon is independent of whether your card allows the charge or not. You can run out of top-up VISA card credit but you may still end up owing Amazon money.
This applies to any other provider, too. What you owe the provider is what you contracted to pay the provider (eg. by consuming services, or clicking the "upgrade" button in a web interface). It is independent of them actually taking the money.
shifting risk to the creditors is the entire point of limited liability; the risk of deadbeats abusing it is always there, and the ability to pay is something almost every business will take into account when arranging a line of credit
amazon could completely negate this risk by requiring pre-payment for small/unknown operators, which is something a lot of people (myself included) desperately want them to provide.
I'm sure they've done their sums here, and have figured out the increased revenue from customers not being able to set a budget is more than their potential losses from deadbeats
the variable costs are basically zero, after all ( bandwidth and CPU time are worthless if not utilised)
This applies to any other provider, too. What you owe the provider is what you contracted to pay the provider (eg. by consuming services, or clicking the "upgrade" button in a web interface). It is independent of them actually taking the money.