It may well be that early testing showed the vehicle to be capable of towing a 10,000 load, but that does not necessarily grant it a 10,000 load rating.
The brakes need to be able to handle it, the transmission often need to have additional coolers, the suspension needs to be able to carry the weight (with acceptable comfort and control). It may be that the without a load transferring hitch that the final rating needed to be adjusted.
Believe a 10,000 capability might be there, but not a 10,000 rating.
Believe that Tesla is doing the right thing here, by giving it a conservative rating right out of the box. Maybe later editions might have higher ratings.
On a more positive note, maybe in the near future we will see the Model S achieve a 3,500 lb rating that will allow towing small boats, jet skis, snowmobiles, moving trailers or camping trailers. They certainly have the low end torque, and might give us even more flexibility with our all electric vehicles.