If you want to eat a whole chocolate bar to get high, you can go ahead and buy a dosed bar appropriately, but a lot of people don't want to do that. And 1 bar might not even be enough as people have different tolerances.
On the black market, things can be dosed so you can maybe do it in 2-3 squares of a 16 square chocolate bar. You can get high, but also don't need to eat an unhealthy amount of sugar. We don't want to be encouraging a chocolate bar a day for some people.