Apparently the r/spacex community got together ahead of the AMA with Musk to come up with quality comments. During the AMA they worked together upvoting their questions to get the most visibility. The mods on r/AMA felt that this was against the rules, and crowded out the natural discussion, they subsequently deleted all posts from r/spacex. Sounds like a good idea, until you realize that they were very well thought out questions, head over heels better then the questions posed by the r/AMA community. I found the AMA to be underwhelming.
Slight correction — it's r/IAmA.
On the other hand it's difficult to make exceptions in these cases. How do you define a comment that is worth what is otherwise regarded as a vote manipulation?