On the other hand: CloudFlare comes off as terrifyingly incompetent here - which more or less matches with my experiences trying them out on a site serving maybe 100GB worth of traffic at most in a month. They seem to have missed dozens of great opportunities to upsell you on their paid service, and when someone finally noticed how much bandwidth you were using, they completely lost the plot.
What should have happened, IMO, is something like this: A panicked CloudFlare admin realizes your site is using 100TB/mo. Their first step is to send you a sternly worded email, explaining that for this usage level you need plan X, and if you don't upgrade within... let's say 5, maybe 7 days? They'll be throttling or limiting your service. Then you don't feel pressured to solve the problem right away, and they are trying to retain a customer that (presumably) they value.
Instead, they haphazardly change your settings behind your back (???) and then later take various steps to reduce your bandwidth usage before finally deciding that you need to pay them, without even figuring out how much money they want. Ridiculous.
To me this just says that CloudFlare is running on what may be a fundamentally unsound business model, and that by claiming their free tier doesn't charge by bandwidth (and not listing any limits) they're dramatically increasing the odds that customers will suddenly discover there are limits after all, and leave. If they were more up front about what the actual pricing structure is, it'd probably be more likely that people would start paying for the value that CloudFlare gives them.