FlightStats has many different types of customers, including developers that use the same underlying APIs that the site does. Some of our customers are really heavy users of historical status and flight alerts, literally monitoring thousands of flights. Other customers have very limited usage of those features.
When pricing these features, FlightStats is trying to find a fair balance that does not unfairly charge too much for the majority of customers with lighter usage, but still charge the fair value of our services to heavy users. You can purchase additional credits as needed.