In truth, Nelson Tobin knew very well why everyone was so wary of using Mandrakes.
It was not merely because the cry of a mature Mandrake was incredibly lethal, and one careless mistake could kill both oneself and those nearby. The more important reason was that they feared other Dark Wizards would learn how to use them too, then turn Mandrakes against them.
Since the Defense Association had been established, Albert had taught them practical techniques more than once, so they could better protect themselves and calmly deal with Voldemort's lackeys. Yet through repeated clashes with Dark Wizards, many of those techniques had been learned by their enemies and turned back against them.
Yes, if they could use a Mandrake's cry to quietly kill a whole group of Dark Wizards, then their enemies could naturally do the same. If they truly wanted to use Mandrakes to kill you afterward, there was no particularly good way to deal with it. They could not exactly wear earmuffs all the time!
As for classifying Mandrakes as contraband?
Living Mandrakes had long been prohibited goods in England. They were not allowed to be cultivated except in certain places. But their relative rarity in England did not mean they could not be found. Even if living Mandrakes could not be found, they could be smuggled in from abroad. There was simply no way to stop that.
Most importantly, once they completely defeated Voldemort and won this Wizarding War, which they had almost no chance of winning, the matter of using Mandrakes' cries to wipe out hundreds of Werewolves at once would definitely not remain a secret.
Thus, Nelson Tobin could actually understand why Albert, even if it meant crossing his own bottom line, wanted to kill every last one of those Dark Wizards. He truly