I'd imagine the main reason is just to promote the idea so people with connections might ask their connections to do an AMA.
Some folks want HN to be sort of a super subreddit.
We already have a site for that, it's called Reddit. I get the impression that most HNers are also Redditors, so doing AMAs on Reddit makes more sense than trying to make HN into Reddit.
If anything I its 50/50 (overlap/strictly-HN), and I think that's being generous.
That was sort of the point, except that it doesn't stop people from wanting a higher level of discourse/participant on Reddit or more Reddit features on HN.