ok all good points -- patents may be involved here, and there is the "crop lens" factor which I'll comment on later. As for just cropping the result of a FF image, yes of course that it trivial. after more thought:
Unfortunately, cropping the FF image does not produce equivalent results to the native crop sensor, at least today for Canon, and there's a good reason for that (at least for Canon!). I suspect there is some FF pixel density, larger than today's 5D3, that would do this at the 1.6x FOV, but Canon chose not to do that -- they chose to advantage the 5D performance in low light/high ISO, and its low-density sensor does that very well. So well, in fact, that it can't (quite) be cropped to 1.6x with equivalent performance to the 7D, which itself is aging. The 5D3 pixel density is approximately the same as the 40D of five years ago -- which means the state of the art has advanced considerably and, should they want to, Canon could produce a 10mp 1.6x crop camera with rather stunning low-light performance. Of course, no one would buy it because of the low pixel count, but it would be doable, and in fact they have a 10mp crop camera today in the 5D3 -- by cropping in post! My point is that at any particular pixel density state-of-the-art, the FFs can always be cropped in post to mimic the crop FOV , but the IQ is just not quite there yet because the crop sensors are designed to maximize resolution in favorable light conditions and will win 'by a neck' when compared with (Canon) FF sensors, which have too low of a pixel density to compete head-to-head when throwing over half of the image information away . For example, Canon could have produced a 38mp FF which (when cropped to 1.6) would have the same pixel count as a 50D, but its low-light performance would not have been as good as the 22mp they chose for the 5D3.
I'm not faulting Canon for this at all; in fact the low light performance of the 5D3 means more keepers and more corner case shots in more difficult situations, greater ability to bring out shadow detail in post, etc. . halleluiah go team go I like it.
Yea I also get that it would be silly to operate a camera body in crop mode when the "crop lenses" won't fit, and I understand Nikon's marketing choices there. But those who would appreciate in-camera crop (and FOV preview in the viewfinder) are the most likely to understand that cropping the image to to change the FOV is not about the ability to fit cheaper lenses on there, it is about changing the FOV (for more "reach") and still maintaining acceptable IQ. I'm talking here about the camera that could truly combine the reach advantages of the crop body with the IQ advantages of the FF. One FF body that, when cropped, would absolutely not be rivaled by any 1.3 or a 1.6 body, when the subject distance and lens is the same.
I do realize that producing such a camera would knock the 1D series out of the lineup because the 1.3 crop bodies would offer no advantage over the FFs of the same pixel density. and it would probably be a $12K body...
I'm not a physicist but would it not be possible to produce a sensor that would operate in more than one pixel density mode (this is a Canon forum so don't fault me for missing something about the D800 here...). I don't mean crop I mean pixel density -- for example, operate the sensor at FF 22mp with all the low light advantages thereunto appertaining, and also at FF 38mp to advantage the situations that the crop bodies depend on -- with favorable light, you get higher resolution when you don't need high-ISO, AND you can crop down to 1.6x for more "reach" and still have a 15mp image that no other natively-cropped body would be able to rival.
By the way, if we are expecting the 1Ds and 1D lines to converge, the only way I can see keeping the birders happy ,for example, is to allow the 1Dx to operate in either FF or 1.3 crop mode -- as long as that offers some advantages the 1D4 crowd will value. But I'm not up on the latest 1dx rumors so apologies if I'm missing a market clue from this design.