Enemy of my enemy. Saudi does bad things against it's own people. While not commendable, it's far better than some countries in the ME that want to do bad things against the US and other European countries. If we alienate Saudi, we have limited our strength to stand against countries like Iran.
And, let's say that is "all about the oil." So what? Is it worth jeopardizing the billions we receive in oil to stand up for human rights against a handful of criminals? Face it, in this world, a human life DOES have a price on it. Depending on which metrics you use (all per capita - dollars spent on law enforcement, dollars spent on healthcare, dollars spent on national defense, GDP, etc.), it still never exceeds $100k.
The US does have a responsibility to stand up for these people. But, the way to do it is to use internal pressure on Saudi (talk to their diplomats, encourage them to do things differently, etc.). We don't want to draw a line in the sand and dare them to cross it.