I'm not sure how it would 'reduce their utility'?
Because the whole point is that you can just hop on one casually to go to where you want to go. As an example, I'm currently sitting in my office and would normally walk home at the end of the day. If I'm feeling lazy I can hire a Voi with the press of a button in the app and ride one home.
I can't do that if it's illegal to ride one without a helmet because I didn't bring a helmet to the office.
Another example: I frequently use a Voi to get to the railway station, particularly if I'm going to London. I'd stop doing that if it meant I had to lug my helmet around London.