I have held back replying to this topic as I have first-hand experience of taking on a former NRM exhibit - so the following is purely my personal thoughts / observations, not in my position as Chairman of Friends of the 502 Group:
As others have indicated, the NRM needs money. Joe Public and family are more likely to spend a few quid in the cafes, shops etc at the NRM than the average enthusiast (who generally would bring their own lunch, buy models mail-order to save the extra 50p it would cost to buy from the shop next door). Although FS has obviously taken a huge amount of cash to repair/restore/rebuild/replicate (delete as you see fit!), the lack of funding has been an issue for a number of years.
So, to try and stop the rot, the NRM entered into various arrangements for different vehicles - long-term loans, sponsorship, transfer of ownership and disposal. The 502 was on the disposal list when the Friends of the 502 Group was formed - if we had not taken any action, it would be razor blades by now. The main reasons that the unit was on the disposal list was not because of its condition, rather that it was never claimed as part of the National Collection originally. It was due to the efforts of members of Steamport that the unit was saved by the NRM - hence when the move to Preston took place and the 502 was returned, there was nowhere for it.
It appears that the T3 was acquired under similar circumstances, so if it is not part of the 'core' collection, then it is understandable that the NRM have transferred ownership.
The bottom line is that the bottom line is more important to the survival of railway artifacts at the NRM than their individual history - which, unfortunately, will be the case until all museums are fully-funded by Government. Therefore, when you next go to York or Shildon, buy an over-priced coffee or FS key-ring and consider it a donation to the upkeep of your favourite exhibit.