Declassifying documents isn't a matter of special treatment, particularly when the government acknowledges the contents have been declassified. And making the information, in this case a two decade old letter, available to the defendents attorneys, who have the necessary security clearances, is quite common. As to it shows how bad his damage was that the bigwigs of the Reagan Administration, no enemy of Israel after all, came all out for throwing the book at him., that might be true. But at the time thoses bigwigs of the Reagan administration didn't know that the assessment of that damage was done by a Soviet spy, Aldrich Ames. Today we do, so it's reasonable to be certain those extrajudicial charges were legitimate by allowing him an appeal. As to the bigwigs, you'll note that although Weinberger considered it a major case at the time, when asked why he left it out of his autobiography, he noted it was an insignificant footnote to his life. He may be guilty as sin, but it wouldn't be the first time government has covered up an error.
>by allowing him an appeal
That has the potential to open a Pandora’s box, and reexamine all that has transpired so far.
I doubt there is ANYONE in the Israeli or American governments who wishes to revisit L’Affaire Pollard.