I don't know about "shouldn't", I think it's fine if they do. But I basically agree, at some fundamental level, you have to have some trust in your coworkers. If someone says "This fixes X", and they haven't even tried running it or testing it, they shouldn't be your coworker. The purpose of code reviews shouldn't be "is this person honest?" or "is this person totally incompetent?". If they're not, it's a much bigger issue, one that shouldn't be dealt with through code reviews.

Very different situation if it's open source or an external contribution, of course.