Its a foolish rule to begin with, why give it credence with pics? Other than make somebody who got the rule into the book feel more important, what is it's benefit? If one were to look at many pics of real folks shooting in prone position, ESP if they are in the field, shooting up and down etc. One sees arms in all manner of positions, but is there some NEED to quantify the angles? Does FORCING conformity enhance the game? I think not.
In the beginning, we did it like hunting ... walking along the lane, and targets were seen, then shooting it in the way that seemed most appropriate (totally optional postions). The lanes were set up in a way the provided interesting and challenging targets that NEEDED one to shoot from different positions, due to the skillful and imaginative settings. Nowadays, in order to make it a "level playing field" its coming down to the lowest common deniminator, with no imagination required .... the powers that be are leaning more and more towards wanting to USE an ACTUAL "level field", featureless and devoid of interest, with little posters telling folks how to shoot, yet allowing folks that mostly prefer offhand shots to ignore the little posters and shoot them all the way they prefer.