Doesn't the employer have the right to fire them for any reason in any "at will job"?
I am not sure if that is true or not or if it applies to google employees, but I feel like there is a ring of truth in it.
While it's harder to get fired here, it is by no means impossible. The law just provides what's basically a three strike system (one to two adhortatory letters for the same reason, then the third time the employee can be dismissed) to give the employee a chance to change behaviour.
Striking employees can be replaced in order to get the work done, but that doesn't make sense for a one-day walkout.
(Edit into this dying thread: reports of the walkout in Google Dublin: https://twitter.com/evelyn_orourke/status/105795687016449228... )
It would be legal to "replace" them with new employees, but that's not practical (or warranted) for a one-day walkout.
Google aren't exactly a bastion of humanistic values, but I'm sure they'd be loath to dispose of a significant part of their workforce while looking fascistic and authoritarian in the process.