You are right about the less frequent usage of the passive in spoken Spanish. We tend to use the "se" form, or the active voice in the 3rd person plural for this. It is nevertheless a grammatical structure used in certain contexts such as newspaper articles. We made "Las rebajas de enero" exercise in a way that shows what you could find written in an article so it is more accurate with the use of the passive voice in Spanish.
However, we are aware of the the fact that it is not widely used in spoken Spanish and we will be making some changes to the content related to the passive to reflect this.
Thanks for your comments.
Un saludo cordial