The location and context of these credentials seems to suggest that they were intended for mostly internal use. Another comment here mentions that the updated client contains just an api_key and client_secret (makes sense), implying that for normal user operations (syncing a user's files) the client probably just communicates with Box's servers, not directly with S3. I guess it would be possible to use a single set of S3 credentials and still lock down per-user access with other means, but I have a hard time believing that's how Box syncs your files.

In any case, this really doesn't seem like the place you'd store user-specific storage credentials (if Box were to use S3 directly in that way). Remember, this was found in a pre-compiled .pyo file, within site-packages.zip.