I feel like interrupting a CEO's speech at a big conference is pretty well understood to be a social indicator of a high level of insubordination. I suspect the protestor knew that too.
The consequences were appropriate, even if I might share some of the protestor's concerns.
Interrupting a speech? Yes. It demonstrates a lack of maturity, decorum, and is completely unprofessional. Someone who pulls these shenanigans is unworthy of the role they were hired for. This isn't high school anymore. They were hired to perform productive work not be disruptive and play pretend activist.
You lost me at "pretend activist". This person put their job on the line for what they believe in, and in a public enough way that complete strangers are discussing it on the internet. That's real activism.
You are trivializing what they did. This is not that they were in a meeting with the CEO and accidentally spoke interrupting him. They started yelling disrupting the CEOs speech at a large event. Name a single company that wouldn't fire someone for that.
US corporate culture has a stronger sense of hierarchy than many other countries. It is an environment where one can get fired quickly and suddenly and that instills a lot of obedience and discipline (if not outright fear) in employees.
The consequences were appropriate, even if I might share some of the protestor's concerns.