in Oracle Fixed assets, why are disposal/retirements done in terms of Cost instead of WDV ?

Normally when an asset is fully disposed we remove its Net Book Value from the books. This is done as follows:

Asset Cost:                                   100,000

Accumulated Depreciation Reserve: 20,000

Net Book / Written down value        :80,000

To fully dispose this asset the cost will be reduced by 100,000 and the depreciation reserve will be reduced by 20,000. Net effect would be removal of WDV of 80,000

In Oracle, we have to enter disposal only in terms of cost. There is no entry for disposal of depreciation reserve associated with the asset.

  If we retire the full cost ie 100,000 the NBV will fall to -20,000.

On the other hand, if we retire cost = 80,000, then NBV will fall to zero but there will be residual cost of 20,000.