Yes, this bothered me, too. R is the most comfortable button for me to dash with (I use the gcn controller), and seeing as I rarely use aim down, it seemed to be the most logical button to leave out. Although that limits my ability to do a crystal flash, and although I haven't gotten into speedrunning SM yet, from what I understand, there are several tricks that require it. =/
IIRC, the site's description of how to do the crystal flash is wrong in that respect. The button combination is "down + L + R + shot" regardless of the assignment of the aiming buttons.