What do you need to know?
Well, they justify it in terms of opposing Iran. For those keeping score, Iran is more democratic than Saudi Arabia, if still terribly authoritarian.
And in case the Israelis get out of line, we're throwing them just enough aid to shut them up. All the Jewish-world-conspiracy folks should take note. As I said in my previous post, it's clear who wields the other.
Whether you like it or not, we're allied to these people. If you're interested in further research, there's an old Orientalist theory called that of hydraulic empire that explained that Middle Eastern despotism was fueled by control of water. We all know what keeps these rulers in power now--oil. It doesn't keep the people in line, but it allows for wealthy foreign allies who'll do the job.
We're less willing to admit that maybe we wouldn't have such a democracy gap in the Middle East if we didn't so frequently, and with such rare exception, come down on the side of those holding the whips. Forgive my tired old leftist lament. We helped the land-owners in Latin America, the torturers in the Shah's SAVAK, and we do the same with this farce of sheiks and princes and kings. Iran, huh? Does anyone buy that?
Right now we give the Saudis arms so that they may keep freedom from coming to Arabia.