I don't, I was genuinely asking. It would seem like given it's popularity that would be an issue. But, crazier things have happened when it comes to consumer safety regulations.

Anecdotally I've probably touched an open flame to synthetic rope, milk jugs and maybe a bucket or two in my life but can't say I've ever seen any of these items react in an uncontrolled or dangerous way.

Sure, they're flammable.

But the amount of BTUs in, say, packaging, is negligible, because it's so lightweight.

When you combine lots of recycled packaging into dense bricks, it becomes a lot of dense BTUs, so can provide a lot of fuel to a fire.

Plastic fires are nasty. Aside from toxic fumes, plastic melts, So gravity will help it spread, if there's a way for it to do so.

You're not going to cause a runaway with a lighter and a pop bottle without working at it. But, say, an electrical short in a closet with hanging synthetics can take off pretty quickly.