14 y/o Muslim student arrested for bringing "bomb" to school[It was a Clock]

[Minimalist]

Yes, precisely.  The legal doctrine is called The Fruit of the Poisonous Tree.

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Fruit of the poisonous tree is a legal metaphor in the United States used to describe evidence that is obtained illegally.[1] 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 fruit of the poisonous tree doctrine was first described in Silverthorne Lumber Co. v. United States, 251 U.S. 385 (1920).[2][3][4] The term's first use was by Justice Felix Frankfurter in Nardone v. United States (1939).

[url=https://en.wikipedia.org/wiki/Fruit_of_the_poisonous_tree]
It does not do the subject of the arrest any immediate good but it does preclude law enforcement from acting upon anything they do learn as a result of their malfeasance. 

It is, in fact, such a serious blow to a criminal investigation that any cop who does it deserves to be reprimanded by his own side.  He just crapped all over the case if there were a case and that is what seems to be the situation in Dallas.  They knew there was no threat.  They were just busting this kid's balls.  Now, we can speculate on WHY they were busting his balls all we like but it is Texas and we have this:

They led Ahmed into a room where four other police officers waited. He said an officer he’d never seen before leaned back in his chair and remarked: "Yup. That’s who I thought it was."

Somewhat prejudicial statement by the cop, there.