The sed command with the -i option lets you delete the contents of a read-only file and lets you delete other protected files. The permissions on a file define what actions can take place to that file, while the permissions on a directory define what actions can be taken to the list of files in that directory. For this reason, sed does not let you use -i on a write-enabled file in a read-only directory, and will break symbolic or hard links when the -i option is used on such a file.