Plates

Question

You explained that you can use the plates at a non-kosher affiar with your kosher meal because they are keli sheni and thus didn't absorb treif taste. I don't understand. Even if you hold that keli sheni absorbs hot taste, and even if we are talking about a keli rishon, why would it make a difference. Since the plates are washed in the dishwasher with detergent doesn't this kasher them, and wouldn't this even kasher a keli rishon. Am I wrong? And if not, then why did you stress the keli sheni aspect?