Mathesar is designed to be used for a bunch of different types of use cases.
I agree you shouldn't allow users to change the schema if you're connecting Mathesar to a production database (that would mess a lot of things up!) We've designed our user roles to account for this – you would assign "Editor" permissions to users, and they can only edit data, but not change anything related to the structure.
On the other hand, if you're using Mathesar as your primary tool to work with your data, it's really nice to be able to change the schema to align with changes in your mental models or workflows.
Schema migrations should go via CI/CD and data changes via your app.
Otherwise it’s gets messy indeed.