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The same reason they mix chemicals with tobacco, to make it burn faster, and so that it never goes out. Haven't you ever noticed that, with tailor-made cigarettes, once lit, they burn until nothing is left, but self-made / hand rolled cigarettes are always going out? It is simply a means to make you buy more cigarettes. To make matters worse, these chemicals are toxic for you and worse than any of the naturally occurring ones in tobacco. This point is NEVER discussed publicly. So of course, the same will happen with big name brand joints.

Ah yeah, I remember reading something like that a while ago. Pretty awful.

That's the future if people don't realise the difference between decriminalisation and legalisation.