It is very easy for CEOs or upper management or middle management to pass down orders that are worded in a way that imply what they want workers to do without saying it in a legally binding way.

Seriously, just think through this. Be super conspiratorial if you want to. There's no upside for Apple as a company. There's no reason anybody in power would even be involved in the speech in the first place. It's a minor awards ceremony that effectively nobody watches. If it were a conscious decision, it would obvious piss off De Niro, which seems like an extra stupid idea.

What's more likely? A: Intentionally anger a big-name actor by trying to force him to change a speech that nobody was going to hear, or B: Someone accidentally sent the wrong final draft.

why is it the first conclusion that De Niro and many others came to?

He said it before he had any time to reflect on it or carefully choose his words to parse out the nuance we're discussing now.