I found this article to be balanced on the subject of returns of artifacts to Italy from the Cleveland Museum of Art: http://www.cleveland.com/arts/index.ssf/2008/11/analysis_museums_often_pay_the.html
Steven Litt, the author, also does a good job getting to he heart of the matter:
It's a field fraught with uncertainty. Before acquiring an object, museums typically contact international police agencies to check whether the work may have been stolen.
The catch is that if an object was looted, there will be no record of its existence. Many museums, including Cleveland's, have collected and shown ancient works whose exact origins remain unknown.
To experts such as Ricardo Elia, a Boston University archaeology professor and a close observer of the antiquities trade, such lack of documentation is proof that an object was looted. He estimated that as much as 90 percent of the antiquities purchased in recent decades by American museums are the product of looting.
But Timothy Rub, director of the Cleveland Museum of Art, said that lack of exculpatory evidence about an artwork's origins doesn't prove a wrongdoing was committed -- or that the work should be relinquished on demand.
"If I've inherited as director custody of an object that doesn't have a provenance before a certain date and somebody says, 'It's ours, give it back,' that's a pretty tough thing," he said. "I've got to ask you to make a case."
The difficulty of arguing such cases makes it unlikely that the recent wave of repatriations to Italy will lead to a vast purge of artworks from American museums.
Instead, if the negotiations show anything, it's that museums, including Cleveland's, are willing to part with antiquities only when foreign governments provide persuasive evidence connecting the works to recent criminal wrongdoing.