After they fired Ameen Najjar, his replacement, Stephanie Hanna, continued to use subpoena power to get info from The Rat.
The money quote:
"Ms. Hannah assumed there was nothing amiss about the arrangement [with Perez] and that it had been completely blessed prior to her involvement in the case."
So they fired Ameen, and then didn't tell his replacement not to do what he got fired for.
It's gotten to the point where you have to wonder not, "what rules have they broken?", but, "what rules have they actually followed?"