The US is the most powerful “nation-state” in the world, at the center of a web of power and finance that extends across the world.
Every single person in the world has a stake in US politics, whether they know it or not.
Insisting that only citizens (or residents, illegal or not) are allowed to have, and express/pursue, their opinions regarding US politics is as foolish as insisting that only the Roman Mob was entitled to a voice in who became Pope.
(This doesn’t change the fact that the US president represents the people of the US, not the world. But everybody has a stake.)