Eh, probably not what he had in mind, but it's certainly possible to make money doing things which have negative net value. You just have to do something which some people want enough to pay you for, and other people really want you not to do. As long as the people who want you not to do it can't prevent you from doing business, you can make money dealing with the people who want you to do it, even if the negative value to the people who want your business to stop is greater than the positive value of the people who want it to continue.

To use an example which was already on my mind, suppose that the people playing drums in the New York subway system (of whom there are many) mostly irritate rather than entertain people. Say that a tenth of the crowd is willing to pay them for their music, four tenths are indifferent, and the other half of the crowd would rather pay them to stop. But the people who want the noise to go away can't pay them to stop, because even if they could cooperate with each other in order to provide a greater incentive than the people who're willing to pay for music, it would just encourage people to blackmail them by showing up and getting paid to not play music.

When a business has both negative and positive demand from different people, it generally takes much more difficult coordination for the people with negative demand to influence the business than positive demand, so the two often do not cancel properly.