That's exactly right: With certain types of materials, you're much better letting it burn off than fight it, to prevent contamination. A lot of hazardous materials are destroyed by high temperatures, so the smoke is "just" smoke--not that the smoke is that much fun, but at least it's not going into the water supply.

My wife and I watch a show called How It's Made, and they covered making those barrels once. Much to my surprise, we're both fascinated by the process of manufacturing various kinds of things. My favorite was fire helmets; hers was saddles.