Wish I hadn't given Straighterline my money

[Merlin]

[...]
You don't have to agree with me. The point I'm making is that it's a simple fix. I just can't imagine having a successful business and still accepting accidental pay. If you can, that's fine for you.

They are a business and they exist to earn a profit. Failing to cancel your membership before renewal is your own fault, not theirs. 
SL certainly isn't the only business who won't refund credit card charges because of failure to cancel in time.

To further expand on this, many companies rely on people forgetting to cancel subscription-based services. That is one reason that these companies don't send out a reminder email or notice to let you know that your renewal period is coming up. I don't know if is part of the bottom line plan for Straighterline, but having worked in subscription-based online games, that is certainly something that we knew about and counted on for our revenue projections. In our case, we'd refund the latest subscription payment and close the account if the service wasn't used and the customer had a good reason, but not all companies will.