Just think about sending a cheque in the post, all your bank details printed on it, a copy of your signature and a covering letter with your address. A huge slice of the population regard that as secure but think the web is unsafe.
I never thought about that. Just about everyday we get a phone call from a customer saying they do not want to use a credit card on our website but prefer to mail a cheque. Not that you pointed this out, you are correct, it is likely an insecure way of paying.