They would tolerate and even want high utilisation by a normal customer; the problem is fraud. As fraudsters want to get the most out of each crime, they are li

They would tolerate and even want high utilisation by a normal customer; the problem is fraud. As fraudsters want to get the most out of each crime, they are likely to hit 100% utilisation. So 100% utilisation is a red flag for fraud, especially if it's on every account at once instead of just one card. It's not really something that the bank can fix by changing the amount of its credit lines. That would only change the dollar amount that each fraud would cost; the fraudsters will still maximally use the credit line no matter its size.