After coaching Agile in many companies, I have got the same question about the role of Scrum Master (and Product Owner as well). Why Managers can be Scrum Masters?
The answer for me is very simple because they are bosses and think in that way. For instance, what happens if the chair of one team member broke? The manager should recommend talking with human resources of the company. On the other hand, Scrum Master has to give his chair to remove the impediment of the team member.
The Scrum Master has to protect the team from interruptions such as managers who want to introduce more user stories in the middle of the Sprint without discussing with the Product Owners. However, Scrum Master should suggest to Product Owner consider very carefully to add new user stories in order to not disturb the Sprint.
Once I had a conversation with a Manager about the rise of salaries from many team members that have got their rise of salary a couple of years ago and they deserve it but the manager responded to me that the company needs to increase its profits before raising any salary. Of course, that manager has annual bonus and other benefits that all of the team members hadn’t. As Scrum Master told him I have to motivate team members and if they are underpaid it would be very difficult to do it. Manager responded to me, well all of them have the chance to move to other company and start getting more money.
Furthermore, another time a tester requested to talk to me because she wants to leave the company. When I wonder why she was considering to quit the company her answer was: the company didn’t care about me and when I suggested to have an honest conversation with her manager she responded to me: I had and my manager suggested to move to other company that should give better opportunities to climb the ladder in her career.
To conclude, I haven’t known any manager who wants to resign his power in order to become a facilitator for each of the team member and I had the same bad experience with managers filling the role of Product Owners.