It's a service--like most other consumer subscription services--where if customers have to take action to keep their subscription going, you're more likely to lose those customers than if they only have to take action if they want to cancel.
My belief is that (1) when people's cards expire, many people don't bother to update their card information in every single place they have a subscription and (2) most people who want to cancel will actually cancel and not just count on their card's expiration date to do it for them. Since we've had just one complaint in more than a year (on the order of 0.1% of number of charges we've made on past-expiration cards), I think my beliefs are justified.
And charging for time spent writing email to or for a client and consulting on the phone sounds scummy to you?