Illegal to attach to bicycle helmet? [View all]

I wanted to attach a homemade turn signal indicator to my bicycle helmet but I saw someone in a video (ironically about attaching a homemade signal indicator to your bicycle helmet) say "Never attach anything to the outside of your bicycle helmet."

I'm trying to find exactly why they said this. Is there a legal issue involved in this? Does it just void the warranty on the helmet? Is it a known danger? All of the above?

I see people who have attached cameras and headlights to helmets, even some specifically designed to be put on helmets. Why would they say that?

Thanks.