While I agree in principle, imgur shows up as an image hosting site, and explosim shows up as a vulgar, racist, obscene webcomic in content filters/ proxy monitors.

It could have easily been done for that reason.

When I was the proxy admin at a company, I had to go to some lengths defending a few workers who browsed to sites the system miss categorized, or overly categorized (think underwear model picture categorized as hardcore porn)