same way placebo still works (to a degree) even when you know it's placebo

your subconcious is not logical, and no amount of conscious logic will fully defeat its influence

to think yourself immune is foolish and dangerous, that's when you allow it to work even better as you "logically" explain away every manipulation you were influenced by, and convince yourself you made a decision fully by yourself. The danger gets even hotter when it comes to political propaganda that uses the exact same tricks as marketing