Yes, a public platform should be used for advocating, but not at the Oscars.
They should write a book if they feel that strongly about an issue or run for office. If they were addressing an audience at a political meet up it would make more sense, but at the Oscars? What does a movie industry event have to do with politics? Last thing people need is a lecture full of virtue signal actors who pretend for a living. It's a downer, not the kind of discussion for such an event, and defeats the purpose of why we're congratulating them. Plus, regardless of the issue it just makes them look bad. I wouldn't want my grocer or taxi driver or accountant to do that. People stop inviting friends to parties if they keep spouting their political views and actors should stick to what they are being paid to do at industry events and save their political views for the right time and place. Like you said, self censoring.
I think I can agree here. I haven't seen the Oscars so I don't know the full context, but it seems a bit of a lame time to be giving political speeches. If nothing else, I would think it makes for boring TV.