I think because I wasn't from an organization that could serve as a guarantee of good behavior. E.g. if I came from IBM and told the students to try heroin, I'd be fired, but as a random individual there was no limit to how irresponsible I could be. (This was before YC.)

It wasn't because of the content of the talk. The authorities didn't see that. The only thing I've written that was turned down because it was too controversial was:

http://www.paulgraham.com/divergence.html

What would you say the limit on how irresponsible you can be is now that you are part of YC?

The fact that it was turned down speaks volumes.

There's a lot of heretical (good) advice in there, so I wonder if that was the reason.