The USA is a democracy, and it is a two-party state. Both parties are beholden to pro-Israeli factions. And the vast majority of Americans strongly support Israel. Justice Goldstone is right, the USA does not have to protect Israel blindly, but there is no significant political opposition to supporting Israel blindly and overwhelming political support for doing so. This is the simple reality. America might not have to support Israel blindly, but no American politician is going to risk the damage that refusing to support Israel would incur.
U.S. weighs sanctioning Russia as well as China in cyber attacks (Reuters)
from the article: Goldstone to Haaretz: U.S. does not have to protect Israel blindly