Although Mendix advocates Agile development in general and Scrum specifically, the company recognizes that organizations differ and there there is more than one valid and reasonable way to skin a proverbial cat. As such, the names of the default user roles in a Mendix project may be derived from Scrum, but at the end of day they are no more than user roles that may be used as the team sees fit. As such, it does not limit users in applying said roles to whomever makes sense.
Adding a restriction to the application of a user role simply because the term is also used in Scrum is like asking why you are not feeding your monkey wrench bananas every day.
Because you could have multiple scrum teams on one project?
When a Scrum team has more experience with Scrum, the appeal on the Scrum Master role for teaching, coaching, facilitating, and removing impediments decreased. Thus it need not to be a full-time job. This opens the possibility for leading of another Scrum team. Also a project could have tasks / stories in multiple delivery teams.