I think it's important to distinguish between limitations on power imposed after the fact, and limitations on power that prevent one from exercising their power. CEOs are not dictators in that they answer to a board, shareholders and, ultimately, the law. But those limits tend not to prevent CEOs from exercising their power, but are rather consequences of exercising their power. On a particular day, when CEOs say something should be done (such as firing a particular person or saying something publicly) it tends to happen.
Whether or not we call a CEO a "dictator" is perhaps a distraction - although I think the word usually fits in the colloquial way people tend to use it. The important thing, I think, is the recognition that CEOs orders tend to be followed, sometimes even against the desires of people executing those orders. The controls placed on CEO's power tend to be consequences rather than immediate limitations.
I found an example of a CEO who fired an at-will employee and as a result the company was successfully sued for breaking an oral contract where the board of directors had promised that the employee would not be fired for speaking about workplace problems. https://www.jacksonlewis.com/publication/south-carolina-jury...
That would be an example of where the CEO is not a dictator, yes?
In some states, a CEO cannot fire someone without just cause, or for reasons motivated by malice or made in bad faith. And there are civil rights, disability rights, etc. which also prevent certain CEO actions. But these can also be viewed as dictatorial government actions overriding dictatorial corporate actions. :)
That's why I brought up union representation and employee contract rights which prohibit the CEO from certain actions.
I think there are also cases where the CEO is unable to impose demands because certain key staff would ignore them or leave the job, or the CEO fears the public reaction should that happen. Eg, http://tvtropes.org/pmwiki/pmwiki.php/Main/UltimateJobSecuri... .