Well now. Here's some passion I must say, I do respect historical accuracy, but I think we're looking for it in the wrong place. This is a science fiction game about alien invasion. I can forgive them for 'continuity errors'. If this was a WW2 historical game, I'd be as up in arms (pardon the pun) as you are.
I'm also not an American, so I don't look at things in the perspective of the States. From my perspective, Shermans were used all over the place for a very long time. And yes, it finally dawns on me that this game supposedly takes place in the States, so yes yes I understand
Just for the sake of knowledge: http://en.wikipedia.org/wiki/Post%E2%80%93World_War_II_Sherman_tanks
All my best,
Pestus