For me, this is a case of don’t hate the player (phone agent, check in agent etc), hate the game (Delta making it part of their jobs to hawk credit cards). Do I find it crass and annoying and intrusive? Yes. But I imagine if I was in their position, I would do the exact same thing and try to excel at my job.

I think it’s good to politely decline and say to the person pushing the offer that you don’t appreciate this sales strategy and perhaps they mention that to their manager the next time they’re asked about how amex sign ups are going.