Because it would imply that we'd given people the wrong advice. The speakers are generally pretty eminent. We'd look bad if we told people to do x and some famous startup founder then told them not to.
I think that any public exposition / debate following such a disagreement would be far more valuable to all present than anything either party could convey as intentional advice.
"Starting a startup is like science in that you have to follow the truth wherever it leads. In the rest of the world, people don't start things till they're sure what they want to do, and once started they tend continue on their initial path even if it's mistaken."
Why would someone with a different opinion be embarrassing?