This is actually what worries me about using markets to incentivize things. The problem is, there's no guarantee that the link between your first and second sentences is true. It leaves room for all sorts of clever Third Answers to bypass what's "supposed" to happen - for example, if Monsanto could get their corn to produce nicotine, and thought they could get away with it for a couple of years, they wouldn't exactly have an incentive not to make their crops highly addictive, thereby ensuring they get more money.
Yeah, but it's to catch those third answers that we have the legal system. You can bet that if they pulled out something like that they would be getting hit by enough lawsuits to cancel out any sort of gains that they'd make. It's one of those things that the people whose job it is to just look at overall legal costs are supposed to catch; that the costs a couple of years down the road are going to outweigh any sort of profit gain now. Additionally trying to pull a trick like that once essentially kills your company. Something serious kills any faith consumers might have not only in your company, but also in the entire field it operates in, so it's in your deep interests to ensure that neither you nor any of your direct competitors pull something serious like that off.
It's totally possible that someone could pull something like that off, and it would suck for anyone affected by it, but the end result is that anyone who does gets forced out or loses in the long run because they are hurting themselves in the long term more than they are benefiting in the short one. There's no way that harming your customer base helps you, so it's in your best interest to always avoid or minimize that when possible.
Note: I'm not saying that they can't hurt people, or that they won't. I'm saying that the force of the market means that statistically, in the long run, the good from any developments will always have to outweigh the bad, or the developments will be destroyed by the market. That's not saying I don't agree with the idea of stricter oversight or similar things, I'm just making a statement that the force of the market is something you can count on in a statistical fashion to protect the consumers from the producers as a whole.