The bots used by these proxies are detectable in a few ways. Remember the bot itself doesn't run on the proxy...

There is discernible lag from proxy to c&c node. The individual bots don't have access to a lot of compute, and are sometimes restricted wrt feature set (e.g. proprietary video codecs).

There are a few other techniques. It's a cat and mouse game though. And the bot owners are usually more motivated than you are.