Trying to think logically about this...
The aim of the exercise was to get the job done quickly. Dropping them live would have meant fitting fuzes, which would slow things down and increase the risk of accidental explosions.
Drilling holes in the casings would, again, be slow and have risks. Steaming out the explosive would take even longer. Why recover the metal from bomb casings in such a laborious way, when there were plenty of tanks to cut up?
There seems to have been an overarching feeling that the sooner such jobs were done, the sooner everyone could be demobbed and get on with their lives...