The lock-down in iOS is more than just with carriers.
The API itself is locked. This is true for a lot of APIs with different mobile OSes. Some make sense while others do not.
For example, on bada, you must have Samsung's permission to use certain APIs because they are sensitive and expose personal information or they consume a service that has to be regulated to make sure that it maintains operations.
So some things make sense to lock down.
On iOS, the media library is locked with no file access at all. Zero. So you cannot open, for example, an MP3 file that is in the user's library. At all. Instead, you have to use their player API to access the media library through URLs. So, if you are interested in using a pre-made crappy player, then it's ok. If you want to do anything, you're hosed.
This is not reasonable. They could very easily simply make the media library read-only. But they don't.
The extent to which iOS is locked-down is just nutty.
"Oh, but somebody could write a program that gets around DRM and..."
I don't see Apple changing things.
Remember, Apple only opened up hardware access on OS X within the last year to allow Adobe to get certain things running properly instead of having to do it all in software, which wasn't working very well.
Inside the walled garden of iOS, there are more walled gardens...