It is called a vendor providing an API for a platform with useful features implemented, so that one can focus on building solutions to one's problems and not reinvent the wheel. Watch out, the way you said it makes you sound like a free software fanatic/proprietary system hater.
I would seriously consider Obj-C on non-OSX systems if it had a better library available (GNUstep doesn't cut it), but that's a chicken-and-egg problem.
It has nothing to do with lock-in. There's nothing stopping people from using Objective C to serve web apps, it's just that no one does it because Objective C is not a pleasant language to work in. People do choose to use Microsoft languages for things other than Windows apps.
Objective-C really is pleasant to use for GUIs. As I noted elsewhere in this thread, I offload everything I can into C++, but between Objective-C and IB, I really enjoy building GUIs in OS X.
