Because he wanted to go?

I'm not disputing that he went. I'm questioning the allegation that he went there in order to pick up moon rocks. There's a big difference between going to Antarctica on vacation and going there -- either voluntarily or by assignment -- in order to collect specimens to pass off as samples obtained from the moon.

Hoax believers want to assign that purpose to von Braun's visit, but to me it makes no sense for him to have either decided to go, or to have been sent there, specifically in order to find and bring back lunar meteorites. Von Braun wasn't the best person for that job, and he knew it. Further, he was a closely-watched person.

When someone proposes something like this, you have to ask yourself, "Is this the best way to achieve the goal?" If, as in this case, there is a clearly superior course of action (i.e., quietly send a team of professional geologists), it is more plausible to suppose that the hoax believer's scenario is an attempt to tack a hypothetical ulterior purpose onto existing circumstances.