Despite the title, overall, I suspect that the numbers cited more accurately reflect changed enforcement priorities than anything else. As was explained to me by an old Customs lawyer, Immigration and Customs Enforcement (ICE) loves these cases because of the publicity. The problem, of course, is that the quest for publicity often leads to hype that is not reflective of reality. Statements like these certainly don't help: "'A nation's culture is not for sale. These are not souvenirs to be displayed at someone's house,' said Anthony Mangione, a special agent in charge of the Miami office of the agency also known as ICE." In particular, this statement implies that collecting itself is somehow an illicit pursuit. Apparently, Agent Mangione does not know or much care that cultural artifacts have long been collected, preserved and displayed legally by Americans in their own homes for generations.
Again, while no one should condone smuggling, preservation of cultural heritage would be better served if there was less hype and more of an effort to incentivize source countries to pass fairer laws akin to the UK's system of Treasure Trove. That way, any problems would be largely addressed at the source than through our criminal justice system here.
A major problem in my opinion is that ICE and other federal agencies rely far too heavily on archaeologists with an axe to grind against collectors and museums as major sources of information. Using activists as experts is patently unfair and no doubt has led the government to overreach on occasion. (See: http://www.accg.us/issues/news/old-pots-cops-paint-as-201chot201d-sold-openly-in-thailand/?searchterm=Thailand) One hopes that the new "Penn Cultural Heritage Center" will not institutionalize this problem, but that remains to be seen.