the reason a coaster brake is easier to skid with is, as everyone's said, b/c you have a mechanical advantage. This comes about due to the differences in gearing.
With a fg, with 70 GI, the rear wheel moves almost 3 times faster than the cranks are rotating. So, it takes a heck of a lot of energy to stop the wheel with the relatively small amount of leverage.
A coaster brake, however, is geared to make it easier to stop. The backward movement of the pedals only has to move the pads of the drum brake up to the surface of the drum. My uneducated guess is that the brake pad, being located as close to the drum as is reasonable, would move less than 1mm for every cm that the pedals move backwards.
3:1 gearing --> 1:10 = much easier!