The height of insanity IMHO.
I love Gluten in my pizza.
People with Celiac disease and severe gluten intolerance can have severe reactions to even an iota of gluten. And a restaurant really can't guarantee that cross-contamination won't occur. This is a disclaimer that most places that offer gluten-free items make. I am sure it's both to warn people with either condition and for the CYA aspect.