Thomas Piketty on Saudi Arabia

"Why would Saudi Arabia choose to invest in US Treasury bonds when it is possible to get far better returns elsewhere? The question is worth asking especially since US university endowments stopped investing in their own government's debt decades ago and roam the world in search of the best return, investing in hedge funds, unlisted shares, and commodities-based derivatives....But the political and military aspect of the choice must also be taken into account: even though it is never stated explicitly, it is not illogical for Saudi Arabia to lend at low interest to the country that protects it militarily." p. 457.