You created a field with calculation rule, so values shall be displayed automatically based on the rule defined on this field. Then you notice that after add this field to the screen, it is read only and does not allow you to edit.
SAP Business ByDesign.
Currently this is the system expected behavior, as the value is filled based on the rule defined for the field. By business logic is not necessary edit this field.
You can also submit your feature request on our Influence page, that is the official tool to propose new functions and share scenarios like this.
Read Only, Calculated Field , KBA , AP-RC-UIF-RT-B , ByD HTML5 Client (not for Cloud for Customer) , How To