It’s also easy to see why the convention would be to not have it. It earns the provider more money. After all, not every customer will ask for forgiveness for a surprise bill.
This is not why we don't have caps. Maybe it's why AWS doesn't. This simply isn't how we make our money. Our top line is dominated by companies growing businesses with us. Getting a hobbyist to cough up an extra $100 doesn't move a single dial for us.
