The real idea here is mentioned in the about page:
http://codingbat.com/about.htmSummarised... a CS lecturer noticed that students who struggle with the small things like syntax, have less time to focus on the big issues of algorithms and data structures. The site gives short practice problems to that effect.
More challenging problems would be interesting, but there's a limit to what you can fit in the stated goal of short problems. Although, I wouldn't say that more difficult problems could be solved in similar space :)