That's not what the F/As are there for. The company pays a lot of money to a sales dept for things like this. Lest anyone think this is not another step toward F/A "grading", watch out. Don't hassle your customers by constantly pushing unwanted/unneeded extra costs.
This reminds me of when I worked for Target as a teenager. I was a cashier for several months before transferring departments. They were constantly on us about pushing Target credit card applications. I was consistently one of the fastest cashiers, but I always ranked near the bottom on credit card applications. When they would push me to do more, I would acknowledge the feedback and not act on it. I hated trying to push credit cards on people who had financial problems. You could tell the ones who could least afford them were most likely to sign up at the register.
That being said, the flight attendants do need to realize that the airline credit cards are likely the reason they still have jobs. Airlines break even, make very little money, or lose money actually flying aircraft. They make the vast majority of their money on credit cards and their loyalty programs. A mandatory training about the credit cards is a pretty small price to pay.