I have not heard of that for workshops. It’s probably part of the liability clause in the venues insurance. The only risk I can think of is someone slipping on a wet floor which is unlikely and preventable. If addressed in the beginning.
It is Ruth. My second venue is within a private residential building and as I am using it for a business purpose I have to do a risk assessment, slips, trips and falls etc, I can see their point. Just a quick run through with my students and get them to sign it. Got to be safe rather than sorry I am afraid.