Your train of logic is ridiculous. People you don't ask to apply for a red card:
1. Person who has a red card in their hand to pay (back in the day this was easily 30% of transactions I did at my store)
2. Teenagers who are clearly too young to apply for one. My store was across the street from a high school.
3. Non-US residents, I lived right on the border. We could not sign them up. I would ask unless I knew for a fact they were not American, I would not guess.
4. Parent wrangling multiple screaming children who just wants to pay and leave.
Sorry, but you are wrong and framing it as discrimination is absolutely ridiculous.
1. Obviously if they have one already you are not going to ask.
2. If a person is clearly a minor, sure, but sometimes you can't tell.
3. If you knew for a fact, fine, obviously.
4. Does not hurt to ask, they can always say no
I've been in retail for over 40 years. When I was a cashier, I always exceeded my credit goals, getting anywhere from 80-120 applications a month. Sears training directive was that we had to ask EVERY customer. It was specifically mentioned that we had to give every customer the opportunity to apply, and not assume that someone would say no, period. This was not my rule, I did not write policy.
Your boss wants you to ask everyone, you ask everyone. If you personally do not wish to do that, well you do you, nice that you have not been called out. Does not change the fact that every guest should be asked.
FYI credit discrimination is a thing, and there are laws against it. If someone were to observe that a cashier was only asking white customers if they wanted to apply for a credit card, and not customers of color, an issue could absolutely be made from that.