This sort of thing isn't going to stop until the individual employees and agents of the government are held personally accountable. They act boldly and recklessly because they can hide behind the shield of bureaucracy, defending themselves with the best legal representation our money can buy.
And obviously this hospital uses a threshold that is set far too low. I have heard of people rinsing poppy seeds to obtain the trace amounts of opium residue, but it takes pounds of poppy seeds just to get a single dose. The amount contained from incidental ingestion of a salad or poppy seed bun would be very low indeed.
This is that special sort of asininity that comes from everything being regulated. There's no more discretion by people closest to the facts, they are overruled by policies that target the lowest common denominator. It's the very definition of mindlessness.
ETA - I should also point out that I think this policy of drug screening all expectant mothers is bullsh*t. If there are reasons to believe a woman is an addict, either from physical exam findings, behavior, or known history, then that's one thing. But across the board testing? They could KMA.