Just to set the record straight, "treason" is defined in the U.S. Constitution as providing aid and comfort to the enemy in time of war. Pollard was not convicted of that. The charge he pled guilty to was, I believe, espionage, which is a lesser offense, but can still (rarely) result in a life sentence. As most people convicted of this crime are not sentenced to life in prison, there must be some special circumstance which is standing in the way of Pollard being released, but I don't pretend to know what it is.
from the article: U.S.: Despite Peres plea, Obama not weighing release of Israeli spy