The reason for this is that /write -d requires the creation of a new file which is assigned permissions by Windows based on the permissions of the folder where it was created and/or the mIRC process permissions.
Most file-related features in mIRC use this same method: if mIRC is updating a file, it first creates a copy of it, updates the copy, and if successful, it deletes the old file and renames the copy. This ensures that if there is an error while writing to the new file, you will not lose the original file.
File permissions are complex - while mIRC could try to copy the original file/user permissions and apply them to the new file, it's likely that there are situations where this would not work unless the mIRC process itself has sufficient permissions. I'll look into this a little more to see if I can find a solution for the next version.
