The least cynical explanation -- and thus the one most likely to be put forward by Apple -- is that by banning cross-platform compatibility layers, developers are forced to focus expressly on the iPad/iPhone platform, which reduces the likelihood of platform-specific bugs and keeps the developer from cutting corners to remain platform-agnostic (such as introducing foreign UI conventions). Likewise, they can claim that by requiring C/C++ they ensure high performance and efficient use of resources, resulting in better a end-user experience.

A more cynical theory: Apple believes they have more than enough developers for their platform, and they need a way to raise the bar. Particularly, they need to limit the pile of me-too apps, and Flash's impending iPhone compatibility threatens to send nothing but.

The benefit to the user in this case is that they have less crap to wade through in the App Store. The harm to the user is that a highly desirable app does not get made for the Apple platform because the developer is put off. And that risk is spectacularly low: any popular app on another platform is quickly cloned, and any killer app that fails to materialize at all... well, the users would never know what they're missing. (The logic breaks down for expensive-to-develop apps, game franchises, and other forms of lock-in, but these cases are few; Apple can negotiate them on a case-by-case basis.)

Because the apps will be running on controlled development libraries that might be able to.... perhaps multitask in OS 4.0, remember guys this is a phone we are talking about here, not a games console, security is paramount, and apple is just trying to put the the genie back into the bottle before a mass of insecure apps are elevated up the permission chain in 4.0 to be able to operate on a par with apple authored apps.

using a different language doesn't affect security, the executable is still within the same sandbox. please read up on the plethora of projects allowing other languages to work with Cocoa, including desktop OSX, including Apple-funded projects like RubyCocoa.