Looks like it just downloads/uploads plain data to the web service. I thought that in this age of internet technologies a web site would be enough.
I personally liked using my own editor, but the CLI didn't provide much added value in my opinion. The tests were enough for me and it was kind of annoying to have to run the CLI every time you're finished just to get the next exercise...
Or how about lynx or w3m?
Plenty of people want to utilize CLIs and it's definitely beneficial when learning to program to get as much experience on a command line as possible.
We already have this great sandboxed environment called a browser for running untrusted code from the internet. They could still offer endpoints for curl, if it's just about using the command line.