Wait... If people can't use proxies to surf with HTTPS, then how does HTTPS provide any security when using Tor? Tor is basically a proxy, and the only reason Tor is safe is because of HTTPS-everywhere. I'm having trouble figuring out why HTTPS works on Tor but not for these kinds of proxies.
My understanding is that you should be able to set up a TLS session with a destination server while you're routing your traffic through a robot-in-the-middle. And the robot shouldn't be able to harm your content in any way, due to the encryption. (I originally said that a free proxy could still cause harm, but if HTTPS is involved, I'm pretty sure that's not true, otherwise Tor wouldn't work. It could drop the connection, but it shouldn't be able to modify anything.)
My understanding is that you should be able to set up a TLS session with a destination server while you're routing your traffic through a robot-in-the-middle. And the robot shouldn't be able to harm your content in any way, due to the encryption. (I originally said that a free proxy could still cause harm, but if HTTPS is involved, I'm pretty sure that's not true, otherwise Tor wouldn't work. It could drop the connection, but it shouldn't be able to modify anything.)