Most people don't say "Not my president" from the beginning of a president's term. For them the phrase is weightier and more significant than it is for people who are always saying that about presidents of the other party. If you are Chicken Little, forever saying that the "Sky is falling" nobody will pay attention to you.
Stealing a bike does not make me a legit bike owner. No one is screaming the sky is falling (a prediction) when they point out facts (from the past).