I suspect this is a real problem for charities, though. If those bots are using stolen credit cards, the "donations" are going to cost the charities money after they pay extra fees to the credit card processors. Nonprofits are sometimes used to test stolen credit cards before making more profitable fraudulent transactions, so there's a real risk of it costing them money if they get rid of the captcha but don't replace it with something sufficiently high quality, even after accounting for the occasional lost donation.