I don't know if it's required, but I believe they routinely do. My GPS consistently shows a few more MPH than my speedometer, and when I've taken long trips I've never gone as far as my speedometer told me I should have gone (based on MPH reported by my speedometer and keeping an eye on mile markers).
Back to the topic at hand, though, locations of police spotted in public locations is public information. This is the type of information that "wants to be free." My father was a police officer, I have several relatives who are still police officers, Several friends of my family are police officers... I highly respect them and, IMO, understand their point of view better than your average person. After seeing all the people lying, resisting, spitting in faces of police officers... I think that they have a LOT more restraint than most people are capable of. While it bothers me that police officers break, it's hardly surprising when the average person hates and distrusts them, despite the fact that 99% of the time they are good people doing their job... it's that tiny fraction that doesn't that makes the news, and colors people's perspectives.
I use waze... and I'll thumb up police reports if I see them, but I never slow down - because I'm not speeding to an extent where the cop would care. Around here the police on the interstates have a detrimental effect on commuter traffic, because when people see cops they slow down to 5 to 10 MPH below the limit, and it pisses me off. I cruise by police doing 5MPH over the limit and have never gotten a ticket. The very rare cases I've heard of people getting tickets for less than 5MPH over the limit have all been dismissed by the court. One department around here (at least) is ordered not to bother anybody doing less than 10MPH over the limit.
I give no credence to this complaint. I know that, in some locations, the police even use waze to see where they're being reported. Then they can move, they can "not there" the report - being a voluntary app (that I've only submitted information for twice because, hey, I'm driving here!) I find most of the reports dubious at best, and most of the time don't even see the cops that are supposed to be there.
The benefits of Waze over something like google maps is not the cop reporting - it's the accident and slow traffic reporting, it's the combined information from 100s or 1000s of "wazers" to help guide you around problem areas. The last time waze told me to detour and I didn't, I got stuck in an hour and a half of traffic I could have avoided.... never again.