The State Department consistently tries to distance the US from the more assertive Israeli actions. The notion is two-fold: first, we're the only ones with leverage over Israel, and second, they're trying to buy credibility with the Arab world. This quote from the Foxnews report on the assassination of Sheik Yassin:

The United States received a threat from Hamas for possibly the first time. The militant group warned in a fax to the Associated Press: "The Zionists didn't carry out their operation without getting the consent of the terrorist American Administration, and it must take responsibility for this crime."

That's the flipside of this policy. If Israel does anything, the assumption is that Washington must have approved it. If they don't believe this, then it's an effort to extract some sort of retribution from the Americans against Israel, or even stronger statements from their friend Mr. Boucher.