Well, the police have dash cams in their cruisers and as of now don't have body cams, so if they are going to issue them they need to get it right the first time. I don't have an issue with them being able to turn off something they aren't issued at present. If and when they are issued it would probably, or hopefully, be used as an evidence gathering tool rather than to actively monitor police conduct. Unlike in tay's case with a 3rd party maintaining the videos, FDR data is as I mentioned, controlled by "gatekeeper" agreed to by both the company and union, and is an employee and union member.
A 3rd party is a disinterested party, and as such may either divulge or withhold information at the behest of one group or the other, but the divulging or withholding would likely be at the behest of the hand that feeds them, the company. I'm not saying that would happen but keeping it all in house with all parties in agreement with how and when this information is released would hopefully keep everyone honest(ish).
I am all for investigating police misconduct, but only when it becomes an issue because of an incident, not as an ongoing campaign to monitor each and every LEO on the streets. Police departments are rife with petty political skirmishes and these folks also have to be protected from spurious investigations by jealous, or zealous higher ups, and without proper safeguards it will happen.
As for an LEO who has their police issued camera turned off and then makes a questionable arrest or some other alleged shenanigans take place I think of the British caution, not the same as in Canada, ( neither we or the UK Mirandize), but it's similar, something like this, "You are not required to speak but it may harm your defense if you do not answer when questioned something you later rely on in court". In other words, if an LEO's camera was deliberately turned off, essentially maintaining their silence, he or she might have some 'splainin' to do when it gets to court. We know that prejudice normally favours law enforcement, this could reverse that prejudice, and rightfully so.