In theory scrum teams should act autonomously and self-determined. An external control by the product owner is not necessary. Of course, he defines what to do, but he does not control how the development team implements the requested features.
In practice this isn’t always true. I have seen and worked in teams which where autonomously and I have seen the opposite, teams with a product owner who wants to define and control every detail during the implementation process. And sometimes there are product owners which don’t do anything and even won’t do a planning meeting or a review.
To find the right balance between autonomy and external control isn’t easy. So as scrum master you will always have the two project partners, the development team and the project owner, which have different attitudes and different expectations about the type of control on the project workflow.
So you can think about this control type as if you have a slider. On the left side there is a complete autonomously development team and on the right side there is product owner who wants to control everything. To have an efficient development process you have to put the slider somewhere in between. But who controls this slider? Someone has to set the slider on project start and maybe during the development process it has to be adjusted. Should the scrum master set the slider? Can he set the slider? If we ask these questions you see that it isn’t easy to define this very important project control task.
So who controls this slider? The answer is easy: Everyone!
Sometimes you have a development team which wants to act very autonomously. So they try to move the slider to the left. Or maybe the developers don’t want to take any responsibility so they try to push the slider as far as possible to the right. On the other there is the product owner. He will also have his personal attitude about the project process and the kind of control he wants to have. And in most cases it is also a question of time he can spend for the project. So he will also try to move the slider to one direction.
To set up the right scrum process is one of the tasks for the scrum master. Therefore the scrum master has to realize this issue and help the different project parties to find the right slider position. As scrum master you should never try to set up a fixed position for this slider. Of course the scrum theory will give you some hints about the kind of team autonomy you want to establish, but at the end you should be so agile to allow a setup which offers best possible work conditions for the whole project team. So as scrum master you have to see whether both parties pull or push on the autonomy vs. control slider and help them to find the right compromise because they will nearly never have the same expectations.