It's possible, at least in the movie-universe a degree or two of "will" on Shaw's part has to take place in order to absorb energy much like Magneto has to use a degree or two of "will" to use his "magnetism" powers, he doesn't just attract all metal everywhere and anywhere whether he wants to or not. He has to make a decision
and even exert some form of effort (either physically, mentally or both) to manipulate metals.
Shaw could be the same way, he doesn't absorb all energy everywhere and anywhere no matter what he has to want
to absorb it and exert some kind of effort to do so. Now, as Magneto has gotten more adept at using his powers over the years from manipulating metals almost randomly during times of emotional outburst, to being able to manipulate small metal objects at will to being able to lift submarines out of the sea, Shaw also has learned to absorb energy almost without thought or "trying" but it still requires some-kind-of thought on his part to do it.
Since Xavier had Shaw under his control during the scene it's possible Shaw could no longer exert any will on his powers so he was pretty much vulnerable to the coin's kinetic energy. (Much like how we see Mystique revert back to her "true form" after being knocked out in X1 or similar times in the movies where someone is knocked out and and their powers stop working.) Hell, the only person I can think of off-hand whose powers don't
require "will" is Wolverine whose powers seem to not be an at-will one but just a very-advanced form of a natural biological process. (Thus, Wolverine heals even when knocked out.)