Best I can tell, Amit didn't actually said he was spying for us, hence the reason the Israeli government has never confirmed as much.
http://forums.anandtech.com/showpost.php?p=30968711&postcount=38
Well, it does say he made a "full" confession.
I would assume "full" means exactly that, not just saying "I betrayed my country" without specifying any details.
Shinbet Agent: Amit, we heard you were chatting with US intelligence officials, is that true?
Amit:..."Insert whatever response you want here".
Shinbet Agent: Can you explain what classified Military and Shin Bet documents are doing in your home?
Amit: I fully confess. I met with so, so, and so in Germany on xx/xx/xx at x:xxPM and every 2 weeks after the initial contact at the exact same time and location and also collected $15,000 during each visit.
If he didn't confess that he was spying "for" the US, so which country do you think he was spying for? He certainly was not spying "for" Israel.
Okay, so you're saying he could have been spying for another foreign country, it doesn't matter same result.
It doesn't matter if he was spying for UK, Russia, China, US, Syria or Saudi Arabia...He should still get the same treatment.
Since it's a fact that he was convicted of espionage in 1987 and sentenced to 15 years in prison(and he never filed an appeal with the Israeli courts or military to prove his innocence).
Possible conclusions:
a.) Israel didn't want to waste their tax dollars investigating a crime that happened after the accuser has "fully" confessed to doing what was already alleged.
b.) Israel silently confirmed he did it, but didn't want to release info or publicly comment because it was in their best interest not to do so. Ever heard the proverb "Don't bite the hand that feeds you"?
As we all know, Israel never comments on security matters so we're both speculating at this point, but I still say what happened is more closer to my scenario than yours.
They rarely ever bother to waste time confirming or denying reports.