I can explain why it puts them at risk.
Over the last half-century of my life, the United States has become a police state. Our local police officers routinely over-step their authority (to put it mildly). This should hardly be news to anyone with a pair of eyes. If you know how to find the thousands of police abuse videos on YouTube, three can be no denying it.
We live in a police state. Consequently, there has been a very rational backlash against police in the last few years. You can see this in particular with the "Black Lives Matter" movement (though their choice of poster children leaves something to be desired, most of the time).
In any case, you can think of it this way:
When I was a young man, if you were getting hassled by the cops, there was some good chance you'd been involved in at least a misdemeanor if not a felony. Today, if you're getting hassled by the cops, it's probably over the city's taxation program.
What cities, counties, and States have done is to turn the cops into tax collectors.
This was not always the case.
When I was a young man, a cop was unlikely to cite you for a traffic violation unless they observed you driving recklessly. The fines for minor speeding, failure to signal, etc, were all very small. I was involved in a three-car accident that was my fault. I was cited for failure to yield right-of-way and had to pay some small amount (the real punishment came in the form of increased insurance premiums).
Today's fines for minor traffic violations now run into the hundreds of dollars, even for the least offense. This is simply taxation by another name.
And the police -- with their citation quotas -- are the tax collectors.
Tell me, do you not cringe if you see a flashing light in the rear-view? Do you not immediately look around, hoping you didn't do anything minor -- because the fine would be exorbitant beyond any reason?
It wasn't always like this. I know it's hard to believe, but before cops became tax collectors, people actually trusted them.
As tax collectors, they are a bane on our existence. This coupled with abuses that are now being captured by anyone with an HD video recorder in their pocket has revealed a truly disgusting side of the police. They're not just a bane on our existence, in some cases, they are actively our enemy,
So bringing all this back:
People hate cops, at least as much as they would hate any tax-collector. Sometimes more.
Disguising your cop car as a private business' car risks detection -- as in this case.
When detected, the natural assumption is that this is neither the first nor last time such deception has been undertaken.
From this point forward, it is perfectly rational to suspect a Google Street View car is, in fact, a police car.
The occupants of that then receive the same hatred as police officers.
This puts the Google Street View car occupants in danger.
If they don't stop tax-collecting, one of these days there will be a significant backlash against police. As tax-collectors, they deserve it,
There's no reason to get some poor Google Street View driver tarred, feathered, and run out of town (the traditional method of dealing with tax-collectors).