I think it's a silly rule.

I think it would be good if audience members could clarify up-front what their contribution would be (e.g. "I have a question/ I have an appropriate experience I would like to share/ I have a relevant viewpoint I'd like to put before you/ I have brought cookies for everyone") but to keep it brief!

I've been on panels that would have died without an engaged and contributing audience. I've been to panels where I've wished the audience would shut up so I could hear more from X.

Let a thousand flowers bloom!