It's that people smoke knowing that it will kill them but that's something different. Think about the dangers of illegal drugs - the chemical itself isn't generally the problem, it's the impurities / bad manufacturing.

If you were buying drugs over the counter you wouldn't expect to hit on a batch that contained a poisonous substance. Drug companies would be very careful to make sure that doesn't happen.

Here's a recent case from the UK:

http://www.independent.co.uk/news/uk/crime/superman-ecstasy-...