It's not like it's all one sided or anything.
The counter example is like it is in some places in the EU. The worker finds a better job but has to give 3 months notice, or something stupid, and by the time he can leave, the other job is gone to someone else. Also, the company can't fire the deadwood in any reasonable time, nor make needed adjustments in staffing in response to business condition changes, so they don't hire in the first place.
All in all, I think that the 'at will' situation is better for both parties. So, no, it's not like you are making it out to be.
In the end, more than anything else, Brandon Eich was a victim of market forces. Mozilla, more than most companies since it's a nonprofit that relies largely on public contributions and its collaboration with Google for revenue, is in a unique position where the views of its executives could potentially be more impactful to the bottom line than most other entities.
Oh, and as far as how this became public, Eich's $1,000 donation met the threshhold for public disclosure in California. The IRS didn't have anything to do with it.
Freedom of speech is not freedom from criticism.