Only the US sells phones tied to carriers. It's very weird actually....and i think almost borderline illegal.
In Asia, you can buy whatever phone you want, and use whomever service provider you want. And, you can actually use multiple service providers at the same time using the same phone (which is what i do). The phones you buy here (including iPhone, blackbarries, etc) come all unlocked (legally as well). So, if i'm in the US, i stick in my Tmobile or AT&T sim card, and it works. In Singapore, i stick in my Sing-tel. In China, i stick in the China Mobile. It's so easy and convient and all bills are paid over the net. And i have a plan where it's all completely pre-paid...so if im not using my T-Mobile in the US, the bill is zero for that month...
the US is about 5 years behind regarding cellphone technology and service. I always feel like i stepped back in time whenever i'm there. If you ever visit Japan, the cellphone tech there is light years ahead...stuff like self-tracking GPS, Video conferencing, etc are just part of the norm on the average phone...
-feng