JSON files can be read by a human in a json-based editor, just as text files can be read in a text editor. I believe they are not intended to be ‘human-readable’ (although many programmer’s would have you believe that they are). They are intended to be restored if your online data suffers a mishap and may contain items that you (or a non-Legend programmer) wouldn’t be able to identify anyway, i.e. “BColor=#FE”
You can export Legend data in several other forms, some more human readable than others.
If what you want is a file that you can read, in Legend native app use OPTIONS/EXPORT and select one of the readable options, such as HTML (to read in a browser), Plain Text (to read in a text editor such as Notepad++), Markdown (to read in a markdown-based editor), or OPML (to import into another app such as Workflowy or Dynalist.
Remember, if you were to move one of these data format files to an app OTHER than Legend, there is no guarantee that all the data would be preserved because every app uses a different method of naming it’s data fields. You may get an outline, but lose all highlighting. You may get an outline, but lose all dates. There is no universal file format for this type of thing, only universal file structures (i.e. HTML, JSON, etc).
For the record, I Backup and export my Legend data in all 5 file formats once a week just in case. It would be nice if it could be automatically performed, but manually doing it only takes a minute and is worth the security being that sync is still so buggy.