Big decisions are often based on a lot of things. Because something happens to serve one purpose neither means that other stated purposes are false nor that the happened-to-be-served purpose was a big part of the decision (although it probably was a major, though not dominant, consideration in the case of Hiroshima).
We also have to take into consideration from where those making such comments are coming. I'm thinking along the lines of those with a hammer seeing all problems as nails, something we have seen more recently (with advocates of air power thinking that an opponent could be conquered, as opposed to just defeated, by bombing alone). Even setting aside personal and career-based considerations, some people simply find it difficult to accept that solutions other than those of which they can easily conceive might be the best ones (for their side, if not for the enemy).
EDIT: As an aside, I wonder if Eisenhower's view would have been the same if he'd have been in charge of, and responsible for, attacking Germany from the east, as opposed to the west. One gets the (perhaps false) impression that Germany's campaign in the east was more of a fight to the death affair -- for political, and probably racial, reasons -- than the one in the west, with casualty figures to match.