[bodski]

It seems the proxies are filtering by URL but all requests to any part of a flagged domain go through the proxy and so the sites see all subscribers of a particular ISP appearing to come from the same IP address. Hence the "your IP address has failed the Captcha too many times" type of messages. The Wikipedia/IWF 'edit ban' debacle was exactly the same thing due to Wikipedia limiting edits from any given IP address to prevent abuse.

Incidentally, I'm with BeUnlimited and it seems uploading.com always thinks that my IP address has exceeded some download limit:

"Sorry, you have reached your daily download limit. Please try again tomorrow or acquire a premium membership."

This does not happen when browsing from a cheap Linux VPS box, looks like Be/O2 are proxying too, except from curl -I output it looks like it's a fully transparent proxy.