We are sorry for the confusion you are experiencing. As AppBuilder is a cloud offering and its use is being charged mainly on a service usage base, all the tools are available as a free download. This lets all users evaluate each and every tool during the trial.
However, being able to use a particular service (in this case - offline development with the extension for Visual Studio) is determined solely on your subscription plan associated with your account. You can see the service coverage provided by the Telerik Platform plans here
and by the AppBuilder plans here
As an alternative to using the extension, you can go with using Visual Studio and the AppBuilder command-line interface, if you do not want to upgrade to a subscription plan that covers the extension. You can read more about the CLI here
Share what you think about AppBuilder and Telerik Platform with us in our feedback portal, so we can become even better!