Because it's Bobblewik. This particular editor is or has in the past been focussed on providing SI equivalents for almost everything that doesn't have them. I had a discussion with him on this particular point, because I feel strongly that "hectares" is what you might call the idiomatic metric conversion for "acres." Bobblewik is somewhat single-minded and determined, but always courteous, well-informed, intelligent, and willing to engage in discussion. I don't want to bother looking up what he said at the time, but I believe he cited chapter and verse for km^2 rather than hectares being the only true, proper, scientific SI unit. Whether it's appropriate to stick strictly to best scientific practice in unscientific topic matter is not so clear. I think I tried to find evidence that _lay_ European readers would be more comfortable with and more easily understand hectares than km^2 and failed to find anything crushingly conclusive. I think the square brackets are very important here. I really detest editors correcting spelling or grammatical "errors" in direct quotations. But Bobblewik is very punctilious about such things. I agree that a footnote would be better, and I suspect Bobblewik wouldn't object to a footnote, nor object to the footnote including both km^2 and hectares... but I don't feel like bothering about this on an article that is not one that I work on actively. I don't see any terrible harm in it the way it is.