In fact, paying with cash sometimes requires a sort of captcha equivalent: the pens[1] that are used to detect counterfeit bills.

Depending on the area you're in, when you pay with a $20 bill (the most commonly counterfeited), the cashier will mark the note with a pen before accepting it.

The business is simply profiling the transaction - a $20 cash bill brings with it a higher risk of fraud - and behaving accordingly. I can't think of a way to argue that it's unfair to the customer that the business does this.

An astute observer will point out that the captcha presented by CloudFlare is an order of magnitude (or two) less convenient than the counterfeit detection pens, but I would argue that this doesn't support a position that CloudFlare is wrong to do what they do.

1: https://en.wikipedia.org/wiki/Counterfeit_banknote_detection...