Earlier quoted context omitted.
Sure, chargebacks cost money. You know what else costs money? When someone wants to give you money, and you misidentify them as a bot and refuse their money.
With donations being blocked you keep sitting at 0, with chargebacks you can actually go negative, in a potentially unbounded way.
For those who does not handle these things: I am not sure on what processor Network Time Foundation is using, but Stripe's $15 fee is actually on the low side of chargebacks (some processors even use the fixed fee + percentage model). Worse, this is unconditional: if you somehow won this, you won't get the chargeback fee.