This then raises two realistic questions:
1) What are you going to do with the artificial joints that do not burn?
2) Do you really want to throw away all of that heat?

Most families, in receiving the ashes of a cremated family member, will not be pleased to find an undestroyed hip replacement in with the ashes. You have to do something with this. What they have done sounds to me like a very prudent move to me. That piece of metal is not really a part of the deceased; it was only an implant.

Secondly, cremation does not simply mean putting a match to the coffin and letting it burn by itself. Considerable fuel, usually natural gas, is expended in a cremation. So there is a definite expenditure of energy from available energy sources to perform a cremation. Once it is hot gas going up the stack, is there some benefit to letting it escape with all of that energy, or might it not make sense to recover as much as can reasonably be done, just as they describe? Here again, I think they are making an intelligent move.