Well, 1> He's usually not "out" as Daredevil.
And 2> Using technological means to listen in to a opponent and their client is an intentional step taken to breach privacy. If they're just in earshot, you've always got plausible deniability at least. From what I recall, he never actually uses that stuff directly in court, because he'd have to justify how he knew that. But it can inform him enough to ask the right questions or look for evidence/witnesses in the right places.