That's part of the story.
It would be the main story if the workers having no rights in the US wasn't much worse for them, and impacted them 100x on their lives, than whatever their businesses do abroad.
About your point, if an international incident isn’t a legitimate reason to fire someone, I don’t know what is. And by extension, what kind of rights you are advocating.
It might be more appropriate to take action against the system that let the error happen. But there are two ways to reduce errors. 1) improve the system, and 2) raise standards for the employees using the system. Cost/benefit analysis ensues.
If you tend to believe him that this was an honest mistake, or even just that there's sufficient doubt, then the conclusion would probably be that he shouldn't have been fired, and his having been fired becomes a question of "how could this happen". And to this the (my) answer is "insufficient workers' rights, allowing a company to take an easy way out at the cost of an individual".
I don't think "letting" is a very accurate word in this case — Marriott could pull out of China (likely at a loss) but the company doesn't want to because it would be financially stupid. In fact, as a public company Marriott is mandated to act in its shareholders' best financial interests.
The reality: You can't take shareholder money out on the front lawn and set it on fire
What the internet thinks: Company not only can do insert shitty thing here but MUST do insert shitty thing here because they have a fiduciary duty.
The problem is that the internet perception of reality is semi fictional. They damn well could have simply stated that they as a company don't take a personal position on Tibet but wont be bullied and faced near zero chance of successful lawsuit even if investors took a short term bath due to business in china.
To refute this simply provide examples of investors who successfully sued because of decisions they didn't agree with an explain how this situation is similar.
Mandated by what law? What if Marriott determines that long-term goodwill in their brand is what's in their shareholders' best financial interests?
But it was the Chinese government that blew a single like out of proportion and threatened Marriott with millions of dollars in lost business. Marriott's response to scapegoat and fire the individual to appease China makes complete sense so long as we continue to allow companies to get away with that behavior. It won't stop until we make an example out of one or two companies to show that they at least need to consider the financial ramifications of appeasement too.