How do you know what you're getting? In SE Asia you got two different grades, High Test and High Test one or both of which may or may not have been laced with high grade opiates.
Why do you keep bringing up purchasing things in foreign countries, as if that's relevant to what happens in the U.S.?
How do you know what you're getting? I suppose one never knows what they're really getting in a product. Do you really know everything that was put into your chicken breast before it got into the shelf (it'll surprise you). All you do know is that stores are in business to make money, and this desire with competition will keep products at an acceptable quality level. If many people get sick from the chicken at one supermarket, you most likely won't shop there anymore. Furthermore, stores are going to want to make more money off of the "strong" stuff, so it's likely they'll mark it and charge more for it. They also won't want to lose their license to sell said product, so it's unlikely they'll do something stupid like lace it with opiates, which are still illegal in this hypothetical scenario. You don't see alcohol being laced with other drugs, now do you?
So in summary, normal commercial regulations that apply to all products (ie, truthful labelling and such), in addition to normal free market and competition forces, would take care of all your concerns in this regard.
If you choose to buy the product off the street by a private home grower, then you really don't know what you're getting. But that's a risk that the buyer accepts when he chooses to circumvent commercial businesses. You can also enact laws that prevent sale of marijuana without a proper license, in which case you can only legally home-grow your marijuana for personal use.
Nope, because homebrewers fall under the auspices of the BATF, who WILL come knocking at your door if you decide to distill spirits (which is illegal to do in the United States, so that might be a better comparison).
I don't think liquor should be illegal to distill, either, but it at least there is a tiny bit of merit to argue that it should simply because it is so concentrated. Marijuana on the other hand doesn't have that type of strength on its own.