Probably because it's a linked system. Usually when you apply the rear, it activates only one front caliper.
I agree with gv, but, most linked brakes don't provide full front when you hit the pedal, nor full back when you squeeze the lever. Does Yamaha provide the same amount of pressure front/back regardless of whether the pedal or lever is used? (with the exception of activating the rear brakes first.)The front calipers are fed with one hose so no way can you apply pressure to only one brake. . . .