I can imagine they don't want accountants or IT people looking at porn while they work though. Their own websites are probably not filtered, but whether the work is for a porn site or a car manufacturer, you prefer them focussing on their work. For a content reviewer though, it's a different story.

That being said, there is not enough money in the world that would convince me to become a content reviewer. The shit they have to sit through is often utterly vile and traumatizing. Here is an article about a facebook moderator as an example: https://www.bbc.com/news/technology-57088382