Hi Am01231,
I answered with my opinion on this earlier in this thread. I personally think that as low as it is … the risk involved with buying a mattress that doesn’t pass the fire regulations would probably be higher than any risk involved in using kevlar thread.
Aramid or Para Aramid fibers (such as Nomex or Kevlar) are used in bulletproof vests and are occasionally used as a fire barrier fabric or as part of a blended fabric but they are more commonly used as the thread in other types of fire barriers. You can see a few comments about them at the end of post #13 here. You can see a Material Safety Data Sheet for Nomex here and for Kevlar here. While they are also synthetic (and some people wish to avoid all synthetic materials), I would consider them to be “safe” as well.
ADMIN NOTE:Removed 404 page link | Archived Footprint 1: nafaa.org/wiki/images/f/f0/Dupont_Nomex_MSDS.pdf| Archived Footprint 2: nafaa.org/wiki/images/8/8e/BGF_Aramids_MSDS.pdf
If you have a concern, as you mentioned, you certainly could remove the FR barrier, which some people do. Realize that in stating this I am not advocating this for anyone, because you are at that point changing the compliance of your mattress to 16 CFR Part 1632 and 1633, and compromising the safety of your mattress as it relates to that standard.
Do some research in the links I have provided and then there will be less “unknown” for you. I fear that you are going down the “rabbit hole” - reading too many things but not having the objective information in order to make an informed decision. But that’s part of what I do here! 
You answered your own question. It’s because it’s the law. And foams are flammable. If you’re interested, you can see a great Chicago Tribune multi-part article here that exposes some of the myths and real motivations behind the 1633 fire retardant regulations
Phoenix.