This decision can be explained as rational only by contorting logic like a pretzel. The simpler “Occam’s Razor” explanation is that this will prove to be a strategic blunder for the Kingdom that amounts to a counterproductive temper tantrum. KSA foreign policy decisions are made by a handful of royalty, the King and a couple of his brothers. There is no bureaucracy to act as ballast to any emotional decisions.
There are many foundational strategic reasons KSA and the United States have had a close and productive relationship for so many decades: because long term security interests converge despite a number of irritants. For the Kingdom, there is only one nation that can secure the Persian Gulf, keep the Straits of Hormuz open and protect global maritime shipping lanes, and deploy decisive force to suppress regional bullies like Iraq in the 1990s and Iran today. The U.S. produces and sells the best military weaponry in the world, and the Kingdom needs it; sure, they can buy alternative equipment from Russia, Europe, or China, but all of these producers are undependable both for policy and sustainment, and their systems are inferior. Saudi Arabia needs the United States as a security counter-weight to Shi’a regimes and untrusting regional rivals. These many strategic imperatives are only trending to be more important to KSA.
The United States needs stable global oil markets; often KSA has stepped up production to stunt the effects of occasional oil production constrictions from producers hostile to the United States and the global oil market. The U.S. also needs the Kingdom to act as a stable strategic anchor for the smaller and varied regimes that make up it neighbors. Saudi aid is a good substitute for certain regimes the U.S. needs to keep at arm’s length. Yet U.S. long term priority interests are swinging toward the Asia Pacific region. As alternative energy sources become more prevalent, and more oil supplies found outside the Middle East, Saudi Arabia remains important but is gradually leaking leverage.
KSA’s purported list of grievances are small in comparison to their long term strategic interests, and are not solved by boycotting their own UNSC seat. It is vitally contrary to U.S. interests to get militarily involved in the Syrian civil war. It was Russia and China that prevented more active UN intervention in that conflict. Ironically the outcome of Syria sacrificing their chemical weapons helps Saudi Arabia as much as it helps Israel: neither are now threatened by this asymmetric threat. The U.S. could not have prevented or changed Egypt’s revolution and counter-coup. Iran had a new election and is making overtures of conciliation with the West and the U.S.; rather than being a threat to Saudi Arabia, literally no state stands more to gain from Iranian concessions than the Saudi King Abdullah. In particular, the U.S. can do more than any interlocutor to make Iran stop meddling in regional Shi’a populations as a condition to reconciliation. Should Iran submit their nuclear material to IAEA oversight, Saudi again gains more security than anyone save Israel.
Meanwhile, what are the Saudi’s alternative to a constructive strategic relationship with the United States? They cannot manage the region on their own. The Gulf Cooperation Council is no NATO: members eye each other with intrigue and suspicion. Will Russia protect them? No, Russia is a self-serving oil producing competitor. Will China tie Iran down? No, China and Iran are partners in international disruption and oil sales.
Like boycotting an election for principles, boycotting a seat in the room means decisions are made while your diplomats are in the coffee shop waiting to find out the verdict: it might feel better and salve some frustration, but boycotters fruitlessly sacrifice their only leverage. Boycotting their UN seat will not change U.S. foreign policy in the directions Saudi Arabia is demanding.
Foreign policy commentators are anxious to explain KSA’s policy as rational and indicative of flagging U.S. leadership. In real life, policy makers (particularly autocrats) make counterproductive decisions all the time, often based on emotion or perceived personal slights. This move by KSA is a good example of how foreign policy decisions can defy strategic logic. KSA needs the US increasingly more than the converse. The Kingdom will probably downplay or walk this approach back in the medium term.