I wonder what the reason is. Is Apple being extremely careful, as they're new to the phone market? Are they trying to avoid mistakes they made with the Newton (this is a wild guess because I'm not very familiar with the Newton)?

It may just be system resources -- either they're too limited, or Apple wants to make sure that the currently-running application (especially if the front application is playing a video) runs smoothly

Another reason may be battery life: Maybe if there's more than one app running - especially if one of them is playing music or video, and one is using the internet - the battery drains very quickly.

I can say that on my Sidekick 3, your application can do whatever you want: constantly read off of sockets, use a bunch of CPU -- but before your application can be downloaded by users, it has to be approved by Danger (and probably T-Mobile).