Well its because you have a linear blur on. Basicaly they cut the tracks so that rotationa and all other movement is approximated as one lienar move between point a and b because most effects do not require more precision. This is the default settting. You can ask for a more accurate multi segmentted motion blur tough and wait for thet much longer (even the linear blur is colstly enough), just like in prman.
Note that the segmented blur doen not work very well on something like a fat rotating weel tough!
Actualy incidenttaly maya default renderer does have one of the most physicaly accurate motion blurs ive ever seen because it does curve correctly and does a stocastic approximation if you use 3d blur, but its also INSANELY expensive compared to all other methods. In fact the best motion blur is porbably the one in prman, wich hands down beats most of the competition, just this alone is a good reason to use PRMAN.
We are not talking of raytrace (now in comparasion raytracing is cheap) expensive here were talking about something that stacks over that expensive here. I mean good curved motion blur wich can tackle all possible problems is about as expensive as hi quality caustics + gi + soft shadows+ final gather, yes combined. Offcourse if you use any of those in conjunction to motion blur then you end up with even more computation wich multiplies on top of this.
Now this gets realy realy hard to do properly if you revolve around many times during one frame. because there needs to be a outrageous amount of samples to get it looking good. And the thing is that a well executed fake is about 1/100000th of the cost. with BETTER quality.
Also you shaders would need to be able to react to motion blur too, unfortunately theres no automatic way for this to happen...
