I think you're right that it's for motorcycles. While I think it would be better than riding without any helmet, I suspect it will be a lot hotter and maybe bulkier. It also looks like the back doesn't come down as far, so it's likely not offering the same level of protection, at least in that area. If you go to any shows/clinics/gatherings that requires an ASTM/SEI certified helmet, it won't meet the qualifications so you could get disqualified. I'm not sure if the buckle would have any big effect, other than being more of a pain to fasten.
Long term I'd definitely look for a proper riding helmet.