No offense, but I hope you've used that site to browse many, many sites, including sites that require logins.
What I'm getting at is that you know nothing about the site you're hitting there, or the people who run it. Since it's acting as a proxy everything you do is running through their server. This means they have the ability to log every URL, cookie, username, password, etc.
Now I don't know the folks at browserhelp.org and they might be fine upstanding eCitizens, but they might not be, and there's no way you can tell the difference from where you are.