Suppose you own a Dell computer, and you decide to replace it with a Sony. You don't have to get the permission of your Internet service provider to do so, or even tell the provider about it. You can just pack up the old machine and set up the new one.

Now, suppose your new computer came with a particular Web browser or online music service, but you'd prefer a different one. You can just download and install the new software, and uninstall the old one. You can sign up for a new music service and cancel the old one. And, once...