They are probably expecting their OS of choice (that is, Windows) to do it's job, since it is marketed as being usable on such combination. For the most part and by itself it probably is, but many desktop applications still have problems, or so I've heard to this date.

2560×1600 is not a bad choice, however, since at 200 % scaling it has 1280×800 logical pixels, which is perfectly normal for such a monitor. If only Windows could reliably tell which applications do NOT support higher DPI displays even though they say they do, they could be just doubled in size and still be perfectly usable, if a bit blurry (you don't get aliasing effects, though). I guess Microsoft'll just have to make yet another API for DPI-awareness to catch the rest of the offenders.