Why do black people always assume other people are racist?

I am not a racist person , This is just something that happen at the store where I work at. So one of the sales associate was having a hard time with this customer lady over a coupon the customer lady went all Karen mode and ask to speak for a manager. I went to check and see what was going on with the transaction when I look at the coupon it was expired mind you the coupon expire on August 20 . I told the lady that I apologise for the inconveniences I never offended or attacked the customer I only explained that we couldn't take the coupon because it had expired date. She called me a racist got angry as to why I couldn't do price change then later went off asked me if her skin color bother me then went off screaming that she was going to report me to corporate for being racist. I was confused at first this woman calls me a racist because I did not take her expire coupon.
Why do black people always assume other people are racist?
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