Please let me know if my understanding is correct:
Prom project setting, user has been added with a role of Scrum Master. He/she shouldn't be able to do sprint related tasks if -
The role of Scrum Master is not listed in the 'Manage Sprint' permission of the project permission scheme.
That is the reason probably we add someone as administrator to make him manage sprint, as that role exist in all permissions by default.
So (if above assumption is right) it would have been better if Scrum Master role was default role in permission scheme (which gets created by default on project creation) so adding person as scrum master role would have been more appropriate.