Non-editable default email address field

Hi to all!

In the WebDesigner, for the section "Contact data", I've changed the "ReadOnly" value in "False" for the fields: FirstName, LastName and DefaultEmailAddress, but the latter remains not editable.

Why can I make the FirstName and LastName fields editable, but not the DefaultEmailAddress field? Is there any logic that allows this?

I've noticed that with some users I can edit the field and with others I cannot. What does it depend on? 

Version: OIM onDemand 8.1.4

Thank you,