iPhone, Apple TV Headline MacWorld Keynote
Ok there are two parts to your question. Network independence is a good thing.... but it would be a little hard on them to expect them to provide both a CDMA (Verizon,Sprint) and a GSM (Cingular,T-Mobile (other freq. of GSM being the rest of the WORLD)). So that they picked GSM I think is the only choice they had since they were going to go global with the product. That being said... why not let you pick T-Mobile or Cingular?
There are two glaring reasons why they did what they did:
1. Price: As you can tell by the people already complaining about the price, there price point was important, and I think they realized they could chop off a 100 or so (I don't know exactly, but think of other phone discounts) by making it with a service plan and giving the carrier an exclusive contract. This reduces price, which from reading comments from many sites, seems to be the biggest gripe so far
2. Features: Visual Voicemail requires cooperation with the carrier. This is obvious. If I had a chance to play with the thing, I think I will find others (I thought I heard something about it "knowing" where it was for weather and maps.... for example). These features means that you can't just design the phone and forget about the carrier, since they are an integral part. Apple went for as many killer featurs as possible. This came at the expense of universal GSM interoperability.
I can see why people will love this, and I can see why some will complain. All I ask is that you stop and think about why things ended up being this way