This is true. But some descriptive facts may provide evidence against a normative theory. The implicit argument was:

1. People often believe that they are justified in undergoing some pain in order to experience greater pleasure.
2. Negative utilitarianism implies that these people are fundamentally mistaken.
3. If (2), then this provides some reason to reject negative utilitarianism.

Of course, the argument is by no means decisive. In fact, I think there are much stronger objections to NU.