I suspect it's because of Google Instant (a feature I don't really like), which has to interact with the server every time a key is pressed to ensure that the results are "live".
If you use the gear icon in the top-right of Google.com to go to "Search settings" and turn off Google Instant, it may fix the problem.
Google isn't the only culprit. When Gawker introduced their much-maligned redesign, their sites also rebinded arrow keys to customized interactions like Advance to Next Story. A few months on, and luckily most of these "features" have been removed.
When UX interactions behave consistently at the OS and browser level across 99% of the web, why would I want to see an unpredictable custom behavior on a certain site? I hold Google to an even higher level of responsibility because their actions set precedents that other sites may emulate.
If you use the gear icon in the top-right of Google.com to go to "Search settings" and turn off Google Instant, it may fix the problem.