It would be up to the details of their individuals contracts. It would be possible for someone's contract to either directly (unlikely) or indirectly (far more likely) allow for debating of politics to some degree. That freedom would be part of their compensation package, though one often not considered in simple models. I think Google is realizing they have undervalued the cost to them of allowing such an action and want to renegotiate. Since it is likely an indirect benefit, this would largely be done by changing corporate rules and culture concerning politics where workers could respond in numerous ways.

While it is possible for a contract specifying 8 hours of work without a single thought other than work (and any legally mandated breaks) in exchange for the given salary and benefits, I've never seen any organization that actually operated using such contracts.