You also can’t just take the positives and activly suppress all knowledge of the negatives, which is precisely what the oil companies did.
Unless your primary goal was to kill the industry, and never really had the victims' best interests in mind at all.
Taking a long view, one could argue that there will be less victims overall. This isn't to say that I'm in favor of making any drug illegal.
Now if you are asking why a company shouldn’t be able to lie and decieve the whole world about how their product will upset the entire global ecosystem then I don’t have the legalese answer for you (although this case should be helpful if you are interested).
The simple answer however is because it is wrong, unjust, irresponsible, causing harm, etc.