Legal precedent - and business model
I suspect that if this sort of discovery proceeded and the results (with regards to how much information is collected and stored) wound up in court, it would make people feel very uncomfortable having these things around. And from a legal standpoint, it would be nice if they were only used to solve murders but these precedents never, ever just stop there. Six months later they'd be combing through your digital records to resolve arguments over lunch money or other sundry trivial matters.