Not always, but it does make it more acceptable, in terms of tone. That's how the power dynamic works.

I don't know. "Punching up" should always be acceptable.

> Just because it was the CEO doesn't make it acceptable

Actually, yes, yes it does. There are some things you can't say to any employee of any rank: racist or sexist harassment for example. And commenting on the performance of an employee that doesn't report to you is also generally a no-go. But legitimate, job-related criticism of the CEO, or any other senior management, is entirely acceptable. Why wouldn't it be?

Yes it is acceptable because it is the CEO. CEOs and lowly coworkers are not the same people and do not deserve the same level of interpersonal communication. CEOs shouldn't make evil decisions and then think they can not have mild criticism laid against them.