Well, these situations usually involve you having to be physically present and won't allow you to be out of office randomly for hours at a time. So that limits the kind of second job you could take.

Also, many people work on fairly locked down computers with nothing much to do aside for (filtered) internet and random "corporate" apps. I'd be quite surprised to learn that in these kinds of situations, where people are required to be physically present to do nothing, they would be allowed to bring in their own computers and openly do private stuff.