Also one of their few friends in the middle-east who also happens to be a big customer for Russian military hardware. But yeah, the Saudi's are engaging in pretty classic cartel behaviour, in response to Iran and US shale: https://en.wikipedia.org/wiki/Predatory_pricing

It gets more bizarre though. Putting aside economics, the US, the Saudis and the Iranians all have a pretty aligned geo-political reason for pumping the oil hard: ISIS.

Politics and war make strange bedfellows.