They have realtime insights into how much read/write capacity or IOPS happens on the majority of their services. Their throttling has millisecond resolution. They have no problem giving you realtime insight into when you need to push the magic button to spend more money and increase your write/read capacities. If billing isn't realtime, it's due to sheer laziness or malfeasance (plausible deniability, my guess), because the data is there.
I asked about this a few years ago. The answer I was given by various friends throughout AWS was consistently that of "oh, our customers don't want that" for whatever that's worth.
This is probably technically accurate. After all, I'm sure Amazon, as any business, weights customer voices by the amount of spend. The people spending millions don't want a hard stop (i.e., kill our production services when we hit a certain amount of spend); the only people who want it are the people spending comparatively small amounts, pre-revenue startups, individuals, etc.
This is an example where being data driven to the exclusion of all else can hurt a company; I suspect having this feature would pay dividends down the road (by being the first to provide a safety net for a startup with a fixed budget that doesn't have production workloads yet you offer a competitive advantage between cloud providers), but the effect is completely impossible to predict or track currently since it doesn't have an immediate impact on revenue or the satisfaction of large, paying customers.
Pro-tip: "the data is there" doesn't mean it's cheap to use.
There's a very simple explanation for this; realtime billing would increase the cost of the product they sell to create something most people don't need.
