Staff users in Ghost are considered trusted because they are allowed to write JS, CSS, use SVGs etc. Anything they need to create a customised rich article design and reading experience is available to them.
If staff user access is opened up to public registrations it would entail a lot of design and UX work both in the admin client and the API to restrict what can be done on security grounds and there would likely still be holes.
You’d be better off using an external system to collect basic content submissions which can be vetted and then transferred into your Ghost site. It doesn’t need to be complex, even plain email would work if it fits your use-cases Increasing levels of automation can be applied if you need it using things like Zapier and Ghost’s Admin API.