Did you purchase the ticket on the Korean Air website? If so, you should prepare the credit card.
It was explained to me like this by the EVA and Interpark Travel Agency staff in Seoul. Interpark is a major travel site here in Korea. I bought a ticket for my friend and Interpark wanted me to prove we were family so it wasn't credit card fraud. I sent them a picture of my ID and my credit card. They approved the ticket.
I called EVA (in Korean, no language confusion in English haha) and they explained to me that, yes, EVA requires a credit card for tickets purchased from their site directly. When you buy from another airline, travel site, etc. the burden is on that other site if there is credit card fraud. That is why EVA doesn't check it because it's not their responsibility. Does this make sense?
So, if you're buying a KE ticket on Delta, Expedia, anywhere, they're not going to ask. The burden of fraud is on them, but typically these other companies don't have the same policy. If you buy from KE direct, they will check the card as it is their rule.