After offering to tip an extra 5% a few times and getting the fish eye, I cynically thought "Oh, wait, he just doesn't want a record of this transaction." The most plausible reasons for that are theft or tax fraud. Sure enough, the IRS's public audit techniques guide for taxi drivers (Google it) suggests that tax fraud is virtually universal: e.g. sample audits on 600 drivers in a city found 600 under payers with an average adjustment of $6,000 (i.e. they understated income by probably $25k+), due to pervasively inaccurate records and, the IRS strongly hints, a culture of intentional avoidance of taxes.

This sort of reminds me of the massive pizza delivery discovered in the embezzlement business that someone mentioned earlier when a shop in Ohio installed a PoS system to track transactions.