It implies that the Americans can find no justification IN LAW for the Israeli position i.e. if America had to act as judge and jury in this case then it would have no choice but to rule in favour of the Palestinian list of claims. Because it is one thing to act as "Israel's lawyer" during an ongoing argument between Israel and the Palestinians. But it is quite another to be asked to be Judge and Jury, because then your advocacy has to be based on the MERITS of the case, rather than be based on need to be seen to be standing alongside your friend.
Two girls killed in head-on collision in Israel's north (Haaretz)
from the article: Abbas urges Obama to impose Mideast peace solution