I heard the LA police tried to institute a two-week waiting period on video cameras after the Rodney King incident.
It should come down to a reasonable expectation of privacy. A cop yelling orders on a crowded public highway has no expectation that other people will not hear (or record) him. It's much different than recording someone's phone calls or using surveillance equipment to record a conversation between two officers in a car with the windows rolled up.
Police departments themselves often use hidden microphones in sting operations, such as drug buys. When talking with an undercover officer or informant, the offender has no reasonable expectation that the person they are talking with can not hear them speak, so it is allowed without a warrant. The same thing applies to phone conversations between police and the public.
Planting a bug in a suspect's vehicle or home without a warrant is a different matter all together. The reasonable expectation of privacy is then broken.
Badly-written laws can and will get abused by the authorities. The court system was put in place as a check to potential abuse or unintended consequences of a law.