Autopilot: Why are some Salesforce fields read only?

Salesforce defines some fields as "Read only".

You can still sync these fields (so you can view it in Autopilot, or create smart lists based on the field), however editing the field in Autopilot WON'T update it in Salesforce (ie it is a one-way sync only). On the Salesforce Mapping settings you will see these marked with an orange padlock:

Note that only custom fields created in Salesforce can be switched from being read only to not being read only. Those fields that Salesforce defines as read only cannot be made not read only.