Sure, a few requests a second from one user isn't much of an issue. A few requests a second from a thousand users starts to become a problem. Someone open sources the code and it becomes a few requests a second from tens of thousands of users, that's a big deal.
It's certainly possible to design services that scale to more or less any load as proved by people like Google, Facebook and Twitter, but it's tricky. As soon as you hit the limit of a conventional RDBMS, even with distributed caching and other tricks, then you're into uncharted territory for a lot of web developers and that means your development costs really shoot up - maybe Betfair decided that the best compromise for their business was to instead simply stamp on people they feel "overuse" their service.
That's their prerogative. Of course, you're also quite free to decide that's poor customer service and you'd rather take your business to Betfair's competitors, and it sounds like that's what you've done - fair enough. I'm just pointing out they might have somewhat justifiable reasons for their policies which aren't entirely based on paranoia. (^_^)