Is that 'free' as in 'the bike is £20 more' or 'free' as in 'the bike shop make less profit on the bike and the helmets they no longer sell' or 'free' as in 'made out of materials that cost nothing by machines that cost nothing operated by people who work for nothing, and given to people who will still buy as many helmets as before'
There is no such thing as 'free'
Cycling has a pretty low environmental impact. I can't see giving people helmets they might not use or even need (e.g. people who already own one, people who got one free with their other bike ...) being good for the environment.
Sharing the road with big, heavy fast moving machinery often operated my myopic imbeciles makes me think about safety. I wear a helmet in case I make a mistake and lady luck forgets about me at the same moment.