Contribution "Fee Amount" field not set correctly with back end Authorize.Net event registration

1. From a contact, click on 'Events'.
2. Click on "Submit Credit Card Registration".
3. Choose an event that uses Authorize.Net as a payment process.
4. Complete the payment.
5. Click on the 'view' link for the resulting participant record.
6. Click on the 'view' link for the associated contribution.

Note that the "Fee Amount" field is set to the same amount as the "Contribution Total".

If you process do the same event registration through the front end, the "Fee Amount" field doesn't show up on the contribution view page. (contribution.fee_amount == NULL).