This is to distinguish them from cracked-open eggs, sold out of the shell, as either just egg whites, or liquid eggs. Under the 1970 Egg Products Inspection Act in America, these products have already long been required to be pasteurized, anyway. There is no choice; you can’t buy them any other way.
Shell eggs are normally sold as is, unpasteurized; pasteurized shell eggs are a relatively new product.
Dr. James P. Cox and R.W. Duffy Cox of Lynden, WA began research into a process to pasteurize shell eggs in the early 1980s. The patent for their process is now owned (2010) by National Pasteurized Eggs Inc. of Lansing, Illinois. The plan is in Flandreau, South Dakota, and the eggs are marketed under the brand name of Davidson’s Safest Choice. A trademarked red Circle P stamped on the shell of the egg marks them as pasteurized. They are shipped coated with a thin layer of wax on the shell, to ensure that no contamination occurs after shipping.
The whole eggs are pasteurized in a warm water bath. They are weighed first, with the weight determining how long they spend in the bath, during which bath the water is kept moving, to ensure that no part of the egg gets cooked.
The white of pasteurized shell eggs will be slightly cloudy, and the whites will take about four times longer to whip. It is also easy with whites from these eggs to go from not-whipped to over-whipped very quickly. If you find them hard to whip, just use the old kitchen trick of adding a small amount of cream of tartar.
Pasteurization of eggs does not change the nutritional value of the eggs.
One may think that contamination can only occur outside the egg, on the shell surface, but that is incorrect. It can be right inside the egg, from the very start, because salmonella may be in the ovaries of the hens laying the eggs.
The American Egg Board maintains, however, that the risk is low: “…if you’re an average consumer, you might encounter a contaminated egg once every 84 years.” 
 American Egg Board. How safe are eggs? Retrieved September 2010 from http://www.incredibleegg.org/egg-facts/egg-safety/eggs-and-food-safety#howsafe
Hoffert, Stephen. P. Pasteurizing Eggs in the Shell: Researchers Take Strides Against Salmonella. The Scientist 1998, 12(15):1. 20 July 1998.