That's not the variables but the application's fields you're refering to.
In fact, when an application is published to a stream, users who have access to the app can only use master items (master dimensions and master measures) created by the author of the application.
With that, Qlik want to encourages the use of master items, so that:
Developer: do all the 'difficul' work; create the app; have control of the dimensions/measures to use, prepare everything
final users: performs analysis, create new chart by simple drag and drops without having to build complex measures.
=> Governd self bi for better results