Yes, there's a difference: US citizens have the right to enter the USA; visitors do not. US officials can deny a visitor entry arbitrarily, and the visitor has limited recourse to appeal such a decision. A visitor who does not cooperate with officials who want to search something is likely to be denied entry.

A citizen may only be denied entry if they're entering the country for the purpose of committing a crime, and if the government can prove that, they'll probably admit the citizen and immediately arrest them instead.

A US citizen cannot be denied entry into the country for ANY reason. A valid US passport is prima facie evidence of citizenship so once you show it the worst that can happen is you getting arrested.