They aren't members of Nato, true. However, they're friendlies. . . . Israel not only isn't an enemy nation, it isn't even a hostile nation.
As far as the U.S. is concerned, Israel has the same legal status as Egypt -- i.e., the U.S. grants them formal recognition as a nation and has agreed to provide direct active military support in very limited circumstances (kudos to anyone who knows exactly what these circumstances are). Neither country is an ally in the formal sense of the word, which means the U.S. has not signed a mutual defense treaty with them in which both countries formally agree to provide military support to the other in the event of aggression by outsiders. The U.S. has signed formal treaties of this sort (NATO, SEATO, etc.) with a number of countries around the world that make these countries allies of ours. No such treaty exists between the U.S. and Israel, so Pollard's supporters have no legal basis for their claim that "spying for an ally" is a less serious offense than spying for anyone else.
In order to be guilty of the actual crime of "treason", you must be spying for an enemy nation in a time of war.
Right. This is why Pollard was never charged with treason.
Obviously you're asking a non-trivial question... but there's one thing we do know for sure: Pollard did not take the information that Weinberger thought he had. Sabramerican also posted a relevant Weinberger quote.