I disagree that it won't harm the prosecutors and police.
The rest of what you said is exactly my point. Those people will get hurt because they now question some basic parts of our legal system. They might violate a subpoena that they mistakenly think is fake and end up on the wrong side of the law.
At the same time, those people are also hurting the court cases that they were supposed to attend.
Prosecutors who can't get their witnesses to come in to trial are going to have a bad time, even if they can later convict those people for violating a subpoena.
The only time it helps them is when people comply with the fake subpoena and talk when they didn't have to... And there's some serious question about the evidence that produces. It wouldn't be the first evidence thrown out because it didn't follow the correct protocols and/or violated civil rights.
And what happens when this all comes to light? Do they throw out every case where this appears to have happened, if it's found to be illegal?
To me, this is a complete negative for prosecutors in the long run.