Your children comments are neglecting the battery cost (it's a lot cheaper to run a camera waiting to record than recording all the time). Device size is important to customers as is how often they'll need to refresh their hardware and both of these things are affected by required recording time.
>I'd taken it for granted that a police body camera would not require manual activation. I'd also assumed that they should be always on. Is there really need for better technology here, as opposed to just a change of political will? When the government in area mandates the technologies use in an 'always-on' way then this is easy to sell, otherwise you tend to find that the enforcement officers give a lot of resistance to being monitored, it's not because they're doing anything nefarious it's just because people don't like being watched 24/7 and they don't want video evidence of them checking their smart phone (because maybe they shouldn't be) or taking a shit (because who wants someone seeing that?) on the job.