When creating new fields, some profiles are checked by default on page load. How are these profiles determined ?
This is actually in the documentation:
When you create a custom field, by default the field isn’t visible or editable for portal profiles, unless the field is universally required.
So, the answer is, if you don't check the Required checkbox, portal users don't get field access by default.
At a more technical level, I believe this means that only profiles with the Salesforce license type will get checked by default.