If you are limited to thesis during your graduate study, what will happen to the development of your skills for research ideas that is not part of your thesis.
The research supervisor must give the candidate a carte blanche to choose the exact (research) question, and to decide on the most appropriate methods.
It is however the student's responsibility to get focus and to deliver the thesis-book. This thesis-book is not just a collection of good ideas. It is proof that the student is able to and has done the research.
If the student cannot "deliver the goods" as a student, the student will not be able to "deliver the goods" after the thesis-book.
I don't know the words I to use to describe a supervisor who stifles creativity of students by putting them in a fixed room, padlocking their rich intellects that could have been harnessed into bringing new breakthroughs for societal progression. Such a supervisor is just a waste of resources and talents, an obstacle to scientific progression.
Usually, a skilled supervisor can accurately predict the horizon of a specific research path. Therefore, the supervisor may have to correct the path of the research for ensuring the value of the research and benefit of the student/researcher under the supervision. These requirements should not contradict with keeping a sufficient research space for the researcher to show his/her capabilities, and contributions.