Why the hell is the US helping Saudi Arabia bomb Yemen?

Why the hell is the US helping Saudi Arabia bomb Yemen? A brief guide. - Vox:

The United States has, for more than a year now, been quietly participating in a Saudi-led war against the Houthis, providing valuable logistical support for Saudi Arabia’s airstrikes. So this missile exchange isn’t something out of the blue. It’s an escalation of current US policy, moving from indirect to direct participation in the Saudi offensive.
The problem, though, is that the Saudi campaign is utterly vicious.