Interesting. I read it as almost more of a cultural assumption (I think - I'm not quite sure how I would word it) than a language related one, but I guess that's probably different ways of wording the same basic thing. I would definitely have had a lot less problem with it if it had been presented as an assumption at the start rather than pitchforked in in a cavalier "everyone knows" sort of way. On the other hand, my default philosophical approach is to be nitpicky about assumptions :)