It makes sense as to why most of the JIRA Agile fields are locked, there is no reason for people to mess with their configuration as they are internal to JIRA Agile.

Epic Link, however, is not. It is a field that is placed on screens for the convenience of linking issues to Epics, it has nothing to do with how Epics work. This field should be able to participate in Field Configurations without having to go through extra steps (https://confluence.atlassian.com/display/GHKB/How+to+unlock+a+Locked+field).