In the US this would be 100% legal. For almost all jobs, you can fire someone for any reason (I don't like your shirt, your jokes weren't funny, it's a Tuesday) or literally no reason at all. The exceptions are few and far between - unions provide additional protection via collective bargaining agreements; you can't fire someone for being in a protected class (i.e. because of their race or gender); and you can't fire someone in retaliation for something like a wage complaint. You could almost certainly fire someone for making a video you don't like (unless they're in a union, which is very few tech employees.)
While it is true that most employment is “at-will” (which means that both sides voluntarily choose to employ/be employed) for both sides, in a friendly employee state like California, there are numerous wage claims and employment law suits that come with doing business. It’s not as easy to fire in some states.
Yea it's legal here in the U.S. since it was at-will employment. They tried to play down the effect of the video and made up some vague performance-related reasons.
It's all good though, I was planning to leave anyways.