There are a very few guiding rules when it comes to International politics. One of them is: If you are asking another nation for a favour, you should also be prepared to give something in return. The problem in this case: If the nation that would be entitled to get something in return for her efforts (U.S) doesn't even dare to ask for an equivalent, can you then really blame the other country (Israel) for remaining "silent"...?? Human beings can do many good things, but they can also be quite selfish. And not only in Israel.
Syrian rebels threaten to shell government forces if prisoners' demands not met (Reuters)
from the article: Israel sees rapid exit from Iraq endangering Jordanian regime