I'd recommend the FTB Launcher, which is somewhat easier to use, is much more up-to-date, and actually got permission from all the mod developers. http://feed-the-beast.com/
While that is technically true (although it would be compiled for the JVM which can be run on ARM).
The Pi has fairly limited resources including memory etc (which minecraft uses a lot of) so I imagine they did have to do some actual work in order to make sure it ran properly.
It's really exciting to see that they're pursuing this from a free standpoint. A lot of people love Minecraft, and anyone looking to learn from an established codebase is going to benefit from this.
The only question I have is whether or not they'll do something with x86 or just let people use the new API instead.
It's only shipping for a platform that isn't quite mainstream, and regardless doesn't exactly have a thriving commercial software market. Fairly low risk; and the publicity should more than make up for any "lost" sales.
It's based on Pocket Edition wich (unless it was added later) is only Creative Mode. Given the limited resources of the Pi I doubt it will have Survival/Adventure.