Yes I am sure it is rated for pellets. In fact they are considered one of the lowest possible penetrators stopped even by something rated to stop a .38 special. Since the shot opens up pretty quickly on short barreled combat shotguns, it is unlikely many buckshot rounds will be hitting the same area consistantly at anything besides contact range, and then the value is harder to factor. However they are still round. Something rated to stop a .38 special will stop a round shotgun pellet even easier. The pellet has far less mass, and is only going moderately faster. The energy and how much surface area it is spread over by being spherical will still make it penetrate far worse.
If insurgents chose to wear body armor and they knew there were going up against US soldiers with rifles, they would go for level 3 or 4 body armor right off the bat or nothing at all. Level 4 body armor can block everything up to .30-06.
If body armor enters the equation most common rifle calibers are just as screwed as the shotgun.
That is very innaccurate for the same reason it does not apply to police. Level 2a armor would be entirely comfortable and not hinder movement or guerrilla tactics much at all. However something rated to stop rifle rounds would severely impact thier ability to perform ...well I hate to call it a mission, but... It would not impede thier tactics whatsoever. So thinking all or nothing is not accurate. Even body armor not rated to stop a specific round will reduce its velocity, and even deflect or stop it at distances if the energy of the round has fallen into the range of something it can stop.
In fact here is a link for you that mentions specificly that it will stop 00 buckshot and most pistol rounds at lower ratings. These ratings are at contact range, so any distance will enhance the effectiveness, especialy against buckshot which slows rapdily in air due to poor ballistic coeffecient.
http://www.bulletproofme.com/Quick_Answers.shtml#1
Single pellets of buckshot must be judged not only on energy, but on shape. Just as a sharp knife will penetrate something with less force than a metal rod, a pointed bullet with all of its mass being applied to a front pointed surface applies more energy in a given area than a sphere where the surface area at impact is much greater proportional to its mass. That means the energy is spread out greater, giving even less penetration. So even a bullet of the same energy would penetrate better. However many bullets rated to be stopped by low levels of body armor have far greater energy than a single pellet, and unless multiple pellets strike the same area that is how it must be judged for penetration.
This is not as the wearer, but as the shooter. As the wearer you would want a greater margin of error and want something that would reliably stop it if multiple pellets did strike the same spot which could only happen at very close range, as well as decent blunt trauma protection. However as the shooter you cannot rely on that happening to stop a threat. Level 1 body armor will stop single 00 buck pellets, because it can stop .38 special rounds with more energy, and a better penetrating shape. So 2a, 2, and 3a should be even greater in effectiveness. Since the nature of a shotguns pellets at impact are too random for general rating, you rate a higher level than needed per pellet for that margin of error for when multiple pellets striking a spot. Also blunt trauma plays a factor. Blunt trauma however is not likely to stop a threat immediately even if it kills later. If you would not rely on a baseball bat to the chest stopping someone with a gun, then you shouldnt be willing to rely on blunt trauma to stop them from returning fire.