I distinctly recall a webcomic in the past few years lampooning cloud storage. There was a guy who said "hey, there's this guy down the street who lets me keep stuff on shelves in his garage." "What does he charge you?" "Nothing, he says it's just cool if I keep it there." and then the stuff is sold off or tampered with, the guy is irate, and the moral of the story is essentially "why did you trust a random guy with a garage to keep your stuff?"
But also the comic as you remembered it has a different moral from the actual comic, and would be very insulting to apply to non-paying dropbox users.
It's okay for a free service to change how they operate with a take-it-or-leave-it offer. It is not okay for a free service to invade your privacy without permission.
Additionally, all users should be able to trust Dropbox just as much as the paying users.
https://xkcd.com/908/
I distinctly recall a webcomic in the past few years lampooning cloud storage. There was a guy who said "hey, there's this guy down the street who lets me keep stuff on shelves in his garage." "What does he charge you?" "Nothing, he says it's just cool if I keep it there." and then the stuff is sold off or tampered with, the guy is irate, and the moral of the story is essentially "why did you trust a random guy with a garage to keep your stuff?"