Why is manipulating users OK for profit but manipulating votes for power isn't?

The difference is that a business is explicitly an adversary; caveat emptor.

Elected officials are supposed to represent the interests of the people. "We choose you to look after things for us."

I'm not saying this justifies business practices, and I'm not saying that any set of elected officials actually do represent their people's interests. But on the face of it, that's the difference.

One aims to illicit a need (which can be immediately fulfilled by a purchase) the other pushes a belief or world-view.

Both are arguably unethical depending on your optics.

Selling something is OK. Fraud is not OK. Pretty simple distinction.