Peter,
The user upload is doing exactly what a user would do manually: the user has to enable the field in their profile before they are allowed enter the date. "Enable" here really means, "Create/edit the data."
If you have set the custom field to not locked, then they can edit it either way the Enable checkbox is set by default. The Enable checkbox behaves just like all the other fields in the user profile:
when the user is Editing their profile, all the fields are immediately
able to be edited (unless locked.)
So, the check box behaves in a way
consistent with other fields in edit mode by being On by default.
If you want to prevent users from changing it, set the field to Locked.
The default of today's date is not actually set for all users. It is just the system default so there is some date to start with when editing it. It is not actually part of the user profile: the actual user data is not created until the data is uploaded or manually entered in the field. This is true for all custom profile fields, actually. When you define a custom profile field and you have 500 users, for example, Moodle does not create 500 new user profile fields with the default data. It does not actually create them until they are filled in via upload or manual profile editing.
The only advantage I can see from having the Enable checkbox set to Off after the user upload would be that it forces the user to check the Enable box if they want to edit the date on their profile. Do you have a specific reason for wanting that to be set to Off after an upload while still allowing users to edit it later if they want?