Ask HN: Why does Google ask for C/Java experience for their front-end positions?

I was perusing Google's current job openings, and I noticed that their candidate requirements for front end engineering include this - :Significant development experience in server-side technologies such as C/C++ and/or Java. and :Knowledge of UI frameworks such as XUL, Flex, and XAML a plus.

While I can _vaguely_ understand why it would be a plus to know those frameworks, can someone please explain to me why they're looking for significant _server-side_ experience in Java and C++? What exactly does that have to do with Front end engineering (since I'm assuming they're mainly developing HTML/JS/CSS applications)? One of many such links: http://www.google.com/intl/en/jobs/uslocations/mountain-view/swe/front-end-software-engineer-mountain-view/

[Apologies if this isn't the right forum to be asking this; I'm just curious.]