Your employer is not obligated to enable or allow you to be spending your work day complaining about them instead of working under the guise of "free speech".
That is something that has been true at nearly every company I have worked at. Amazon is not different in that regard, at least not on the teams I was involved with there.
There are a lot of social events at Amazon. I did not spend my work time approaching random people at their desks to mouth off to them. That would be weird and stupid. I simply shared it with people during social conversations at social events where we are encouraged to interact with others.
During social events, I am quite sure that free speech covers what I say.
And in these cases, when challenged, they have been found to have the right to do so.
Free speech rights regard the ability of the government to restrict your speech, not your employer.
Free speech rights also do not come with "freedom from consequences of such speech".