Actually, that's not true. In all of US history, there have been less than 40 treason prosecutions, fewer convictions, and even fewer executions. Many of those prosecutions are related to the Whiskey Rebellion, Thomas Jefferson's Embargo Act, and the Aaron Burr conspiracy. The Rosenbergs, for instance, were convicted and executed on espionage charges, not treason charges.
So, Pollard would not have to have been changed with treason to be subject to death, or have the sentences changed since then?