RASHI (7a, DH Chayav) writes that the reason why the area above ten Tefachim is not considered Reshus ha'Rabim is because "the public do not dominate there." It seems that Rashi's intention is to say that since Reshus ha'Rabim is defined as the place where public activity takes place, since the bulk of public activity occurs within ten Tefachim of the ground (people only rest their loads and perform their other activities within ten Tefachim of the ground; in addition, as one's hands are the center of his activity, and the average height from the ground to a person's hands at his side is ten Tefachim), this height is the extent of Reshus ha'Rabim. (Even though people walk and their upper bodies are higher than ten Tefachim, nevertheless they do not place their objects above ten Tefachim, and therefore above ten Tefachim is not an area in which the public dominate.