Obama appears to be claiming that the USA *must* veto so that it can "preserve" these USA-mediated peace process. Yet if the USA *does* veto then the USA will be proving to everyone that it is not an "honest broker" at all but is, instead, merely "Israel's lawyer". The Pals will obviously have no choice but to spurn any further offers of US mediation and look elsewhere for a mediator, so the veto will actually kill the very thing (US mediation) that the veto is supposed to protect. So why do it, unless you like proving your own irrelevence by making pointless gestures that reveals your own utter nakedness?
Poachers kill four rangers, army officer in Congo park (Reuters)
from the article: UN set to vote on settlement resolution; U.S. set to veto