I'm guessing they subscribe to a blocking service and can choose the "categories" that they block. The categories are entered by humans and sometimes there are mistakes. So, for example, they might block "malware" sites and freecycle might have been hacked at one point and had some malware. Even if it was fixed, it could still be listed under "malware".
Anyways, try the ZenMate plugin for Chrome. I like it a lot. I use it for privacy since I'm pretty sure higher ups could pull my history and may not react favorably to MMM.