That's actually a really depressing thought. Now we'll all have to start being even more vigilant to make sure that the "checkout" buttons are "start the multi-step checkout process" buttons, and not "gimme this item with whatever payment information is on file" buttons.
On the plus side, most sites I'd be really worried about probably don't have my checkout information anyway.
Really? Are there really so many sites that you don't trust, yet have they have your credit card info? I can't think of a single site that fits that bill for me. If one did nefariously charge me for something an make it a hassle to cancel, a chargeback would be an easy solution; it's not likely I want to maintain a business relationship with them anyway.
On the plus side, most sites I'd be really worried about probably don't have my checkout information anyway.