Yeah - but they pull that same shit all the time. That was the hook the both House of M and Age of X; that was the hook with Secret Wars and Age of Ultron. The point is - if "reality is changed" then it's not the character we know full stop. From there we have two options - either they change the character back or we learn to love the new character. That's definitely not happening in this case.
EDIT: Im not saying this is a great story or anything of the sort. I'm not really equipped to say that because Im not reading it. What I am saying is that the criticism whereby "this is not something Captain America would do" is the central concern is unfounded, because this is not the Captain America we know, this is a fundamentally different Captain America.
I don't think comparing it to Red Son is the right approach because that story was about a Kal El that found morality in a society that was fundamentally different to what we knew; Superior Spider-man isn't an appropriate comparison either because that story was about why "Spider-man" is more than just a face and a name. This story is about faith in ideals, misplaced, misguided and otherwise. That there is an "evil" Captain America at the centre of it shouldn't be too hard to comprehend - he was changed by magicness specifically to be Evil. Every single part of his history was altered so he would be an evil Hydra guy by a magic thing that can change reality. This is not the same character.