the reference to @fields is intentional.  it allows supporting the legacy json schema and if it's in use it ensures that the resulting riemann field name does not start with 'fields-'.  this means the resulting riemann field name is the same whether the legacy or new json schema is in use, which prevents the need to change your riemann configs should you migrate from the legacy schema to the new schema.

i'll add some more documentation and comments to hopefully clarify things a bit better.