It is not that Delta is attempting to charge jumpseaters on mainline, it is more of a glitch with the software.
Here's how it goes:
SkyWest and ASA are the first to go to a mileage based fee (other DCI's to follow) for non-rev travel.
When a SkyWest/ASA pilot checks in with a J/S request the computer will have them in the cockpit J/S....no fee.
If the flight is NOT full, or there are multiple J/S requests, the gate agent will move the pilot to a cabin seat and and then the computer will put you on the non rev STBY list according to your PPR number (something a FedEx, or AA pilot does not have). When you are added to the STBY list you are automatically a "non-rev" and no longer a jumpseater.......therefore you will be charged for the flight.
Delta is supposedly working on a fix to this so that if a jumpseat request is made the non rev charge is deleted.
As long as you are riding in the cockpit jumpseat you will not be charged currently (as of Dec 04th when the fees apply).