We do that so people don't have to go about the process of downloading and installing the correct version of arduino, downloading and installing the drivers, downloading and adding the library, and having all the extra Arduino libraries and examples there to confuse them (especially since there's an Arduino robot in there now). We're mostly targeting non-technical users with Sparki, so we wanted to make it as easy as possible to get started.
If you'd like to just have the library, we have that available on the starting page here: http://arcbotics.com/products/sparki/start
Here's the most current version of the arduino library at this time: http://d3k8ss0l8daviq.cloudfront.net/Sp ... .0.5.5.zip
I haven't used codebender before though, so that's an exercise for the user