J-1 Visa abuse is a serious problem. By serving as an entry vehicle to the US, holders avoid the hazards of using the alien smuggling infrastructure, as well as potentially triggering suspicion at the border crossing. The abuse comes from holders who do as Masood did, and overstay their visa.
J-1 visas are intended to be a temporary permit. Holders are expected to return to their home country for 2 years after expiration. Only after that, will they be permitted to apply for entry to the US.
Unfortunately, waivers are available, and many use them (I'm trying to track down the exact numbers).
I found some estimated numbers:
From FAIR - Federation for American Immigration Reform.
The most recent estimate of the INS is that about 40 percent of the nine to eleven million illegal alien residents in the United States originally entered the country with nonimmigrant visas.
The GAO - Government Accountability Office has a plethora of documents relating to visa overstay abuse. As I'm packing for a week's trip out of the country (no visa required), I have no time today to root around in their files. When I return, I'll make a point of delving into the figures, and write a post about it when I finish.