I remember when I first started working UA flights, there would be a pretty sensitive fraud check system. If the system decided that there was potential fraud going on, you had to present the physical card to a UA counter, or someone did (if the owner of the card was in a different city, they could bring it to a UA counter and any agent could release it). The only other option is to refund the ticket, with the option to re-purchase it at the same fare with a credit card you have.

Pain in the butt, often done with teenagers flying on a ticket their parents bought. Thankfully this basically disappeared a few months after I started