Ok, so here are some counter arguments to cop cameras:
1) Officer discretion is gone. Jay walking? Have a tiny amount of pot? Prosecute everything since it's on camera and cop might conceivably get into trouble if he lets it go.
2) Potential for privacy invasion. The cameras don't just record the cops actions, they record everything in their line of sight. 800,000 cops in the US = 800,000 cameras on the street and inside people's houses with data stored on government servers.
3) Slippery slope. If you can put camera on cops, why not put them on other government employees? How about post office workers - mail theft is a serious crime. How about private sector employees.
4) More criminals let out on a technicality. This footage is a gift to the Saul Goodman type lawyers who can now pore over every single thing an officer does or says.
5) Cops are people too. How would you like wearing a camera on your job? Would you behave differently? Idk, I think this has subtle implications on good officer retention and also performance as they avoid every even smallest risk in everything they do.
This is more or less off the top of my head. There are probably many more. I'm not even saying we shouldn't do it, but it's ridiculous to say this is obviously a good idea, no discussion necessary.