FWIW, the same thing holds true on award flights. My family flew US Airways PHL-MCO last summer on award tix. On the outbound they had mechanicals so they rebooked us on NW thru DTW. I had all of our WP #'s on my PC so I called NW and got our WP #'s into the records. Original booking class on US was X (award), on NW it was Y.

On the way back they were oversold by 4 and offered 2 free RT's per pax. We got 8 RT vouchers and reaccomodated on NW through MEM. Again got our WP numbers into the PNRs.

All in all, we only got about 60% of the legs credited. So NW must have had some way of knowing they were not normal Y tix. I did not call to complain as I decided 60% of the miles was better than none.

I have to say I was very impressed with how US handled the two incidents, when flying on an award they do not have to reaccomodate you by buying a ticket on another airline. Then again, I am top tier and give them a lot of biz.