Truth is most people's APIs really cost us nothing to run, so we'd rather not charge for it :P
I'm a really big believer in sites that provide a ton of utility for free, and only charge you when you're actually costing the company something.
That said if you do want to use a ton of CPU, that gets expensive for us - so we may make paid accounts for people that use CPU heavy API's. We realized Blockspring would still be a lot cheaper than keeping your own EC2 server up all the time, so that's sorta cool!
Also if you wanted it set-up locally at your company and 24/7 phone support (from the two of us), we'd have a paid account to cover our time.
