An Exercise In Circular Logic

This is a topic I've covered before, but there's a particular aspect of it that just boggles my mind and I really, really need to vent it out before I lose my mind. Just because the phrase "circular logic" features the word "logic," doesn't mean it's good. It's awful, hearing it makes me want to vomit in rage.
Circular logic is when premises in an argument are used to verify themselves. "Billy is healthy because he eats vegetables and he eats vegetables because he's healthy" is a pretty shitty example, but you get the point. It starts nowhere, ends nowhere and proves nothing. Unfortunately, this particular kind of fallacious reasoning gets used all the time in discussions about a certain touchy subject, so here I am, standing before the tanks, screaming "No! No more shitty logic! This is my last stand!"

Marijuana is illegal because it's bad for us.
How do you know it's bad for us?
Well, it must be! It's illegal after all!

I'm not joking, friends. This is a line I've heard before. More than once. Frankly, once would have been enough to melt my brain, but I'm slowly coming to respect the overwhelming stupidity of people in my old age, so it took this aural diarrhea a few attempts to thoroughly make me want to kill people. Obviously I'm betraying my stance on the whole issue here, but there is a point here that isn't just "weed should be legal," and that is:
The legality/illegality of something has no bearing on how moral, how correct or how healthy something is. I mean, let's stop and think for a second. Cigarettes and alcohol remain legal, even though their effects on the individual and, in alcohol's case, society, are demonstrably worse than the effects of marijuana. A consensual polygamous relationship is illegal, while adultery isn't punishable by law. Obviously, I don't think the law has any place in telling us what we can or cannot ingest/imbibe/otherwise take into our bodies, and in the case of (consensual) sexual relationships, it's really an issue to be solved between people, but if your argument is really that the government is keeping us safe and healthy... it's flawed.
And that leaves me where? Well, right now, not really caring. There's no reason for me to actually worry about this issue in general, but I do worry about people's poor grasp of logic. The world would be an infinitely better place if people actually made sense when they argue something, rather than just spewing the first biased idea that springs to their mind.