Two things here...many guys (not you) have a bear of a time getting the nut loose. When they put the thing back together they don't want to have that problem again so they don't worry about torquing it. They put it on to what seems tight and go from there. The key on the crank does not hold the pulley, it indexes it to the right spot and the nut, squeezes the "feeling" out of the pulley, so that it is too numb to move...thereby holding it into place, Therefore, if you don't torque the nut, in about 50K your pulley will wiggle on the crank and your pulley and maybe crank will be ruined.
#2...you can wrap a tie-down strap around the crank, then hook it on the frame and when the pulley turns it will tighten down and keep the pulley from turning while you crank on it.