It's the same as people who don't value something they regard as rubbish and are pleased to get someone to take it away. The moment they discover that the "rubbish" is being put to good use, they either want a fortune for any more or want something for what's already be used.

Had Elgin not taken a lot of what he did, it would have been destroyed because the Greeks had already used some of the limestone statues from the Parthenon to make lime (mortar?) for building.