This is far more pervasive that just minimum payments. Behavioral economics research has shown the power of defaults -- make a group or policy opt-out instead of opt-in and you'll get a lot more people to join it, even though the two situations are technically identical. Seems to be the same psychological principle at work here.
I think that's just the result of people not closely examining the page and leaving a box checked, vs. deliberately checking it. Requiring users to do anything will almost always have a lower conversion rate than requiring them to do nothing.
