Except the story is that, as part of his ultimate revenge, the villain has unstuck Cap in time, forcing him to live D-Day over and over again, which means he's trapped in the era where--as is totally canon--he punched Hitler. Your arguments would hold true for pretty much any other superhero except Cap!
Actually read the series, and you'll see that Cap's role is far from being cardboard cutout. You seem to be judging this from the out-of-context scan, rather than its place in the storyline. Brubaker's shaping up a pretty amazing epic, one which has already contained some of the very best Captain America stories I (a DC boy, mind you) have ever seen, and are making me want to go and track down earlier stuff like the renowned Gruenwald run.
Seriously. Brubaker has definitely earned my trust that he's going somewhere with this. The reason why Cap's even in that period now is the result of (as someone else put it), "grade-A villainy."