>The little problem is all the people who might have Googled 'how to set fire to a car' but who didn't actually set fire to a car.
That's not what they did. The got a warrant for those that searched the specific address around the specific time the crime occured. Then tracked the phone to the crime scene. After that level of identification then they subpoenaed the searches for that person.
Precisely. This is similar to the case of the catching of the Golden State Killer, where they used DNA correlation to find a possible suspect but that narrowed the focus of investigation; it wasn't the evidence brought to convict.
I think it's probably acceptable for society to grant large leeway for information that focuses an investigation, while keeping the set of acceptable evidence for conviction narrow.
That's not what they did. The got a warrant for those that searched the specific address around the specific time the crime occured. Then tracked the phone to the crime scene. After that level of identification then they subpoenaed the searches for that person.