I never worked at Google and I probably never will. Still, if a boss tried to censor/alter my speech I do in my spare time using my own equipment and connectivity, I wouldn't like it.
This is literally every company in the entire world. How you talk about your own company is something you have to be mindful of because you are an agent of that company; the words you use and the way you react to things can tip investors off on how things are going internally. Do you think anyone wouldn't be mad at you if your company released a brand new product and you hopped onto Twitter to say it is complete garbage or something?
People often take statements from employees of companies as more authoritative than they are intended. If you are going to comment on something people are likely to make incorrect assumptions about and you know that, it makes sense to clarify that. In the case we're talking about here, it's better for both the employee and the employer.