Simple - really. All aircraft braking performance numbers are certified/predicted without reversers. If you've got them fine - if you haven't - also fine.
Actually, some operators on some planes do use the reversers when figuring performance on an icy or cluttered runway. Not a factor with Spanair but it was commented on in the NTSB report of the Southwest crash at Midway:
The Safety Board said the pilots received mixed braking action reports for the landing runway. The flight crew used an on-board laptop performance computer (OPC) to calculate expected landing distance. They entered multiple scenarios, including wind speed and direction, airplane gross weight at touchdown and reported runway braking action.
Observing OPC indications that they would stop before the end of the runway with either fair or poor braking action, they decided that they could safely land at MDW. However, the pilots were not aware that stopping margins displayed by the OPC for poor runway conditions were, in some cases, based on a lower tailwind component than that which was presented.
Also, the pilots were not aware that the stopping margins computed by the SWA OPC incorporated the use of thrust reversers for their model aircraft, which resulted in more favorable stopping margins.
Therefore, the Safety Board concluded in the report that had the pilots known this information, the pilots might have elected to divert to another airport.
NTSB Decides on Midway Crash | Air Safety Week | Find Articles at BNET