Without a warrant, most evidence (with complicated exceptions) can't be admitted into court, but can aid police in solving a case, then using "parallel construction" for court.
While relevant to the topic, I don't think this is responsive to the question. I.e., "why are police allowed to do this at all?" differs from "is it allowed in court?"
This touches on the idea of "poisoned fruit" or, more specifically, the "fruit of the poisonous tree". To quote Wikipedia:
> Fruit of the poisonous tree is a legal metaphor in the United States used to describe evidence that is obtained illegally. The logic of the terminology is that if the source (the "tree") of the evidence or evidence itself is tainted, then anything gained (the "fruit") from it is tainted as well.
The idea here being that the data obtained from hemisphere isn't considered "illegal" and, honestly, probably _would_ be permissible in court. The reason it's kept out seems to have more to do with optics than legalities.
So, I guess the simplest answer to the question of "why" is "there's nothing stopping them".
I disagree. I think it is kept out because it violates your 4th amendment rights, and if enough judges saw that violation at least one would remember their civic duty.
