I disagree. A manager should be skeptical to ensure everyone is pulling their weight and being truthful.

If they don't trust the employee they need to fire them.

I agree with you that employees that you cannot trust should be fired. Skepticism doesn’t necessarily mean lack of trust. It’s possible to trust dishonest people and without a skeptical view to be aware that people may be dishonest, even in small ways, it is easy to fall into that trap of trusting good liars.