I would place a hefty bet that these members are in the 'heat of the moment' group of visitors to your site.... someone whose mom, wife or husband was out for errands, etc... opening a perfect, but short lived, window of opportunity for a little personal time.
I also think you're 'right on' about monitoring new membership activity for a reasonably accurate way to intercept a charge-back.. question:
How will you do it? If the member hasn't logged in, will you simply cancel the membership yourself?
Thanks for sharing your brain waves