Boiling lessens drying time, which is further cut down by heavy metals, spirits etc which can be added to commercial products as John said. Bad for spoons, bowls, chopping boards etc but great for other woodwork. No linseed oil, raw or otherwise, needs to be boiled before use - the issue is that a good finish can take a very long time to get with raw oil.
I believe though that the chemicals aren't limited to boiled linseed oil. A shop near my work sells raw linseed oil, which I've considered buying, but to be honest I don't believe for a second that it has no chemical additives, and since it's not supposed to be consumed, they're under no obligation to list the ingredients I don't believe.
Robin Wood did a good post regarding it on his blog I think, might be worth a look! I believe, though can't be 100% sure, that he favours walnut oil for the work he doesn't sell and imports pure linseed oil for what he does sell. Being a green wood worker, I figure he's a good authority on it
Pete