Nearly ALL pallet/dunnage processing is done with HEAT not chemicals. Not to say I have not seen any treated, but like maybe one in 2-3K. The stamps are ususually indicative of the "required" stamp granted (usually burned into the wood) by either our Dept of Ag (I cant remember which bureacracy I spoke with about this), or USFS. You basiscally run a "batch" in your kiln with their control monitor device, then once they are satisfied with your process you simply sign an agreement to process ALL stock in the same way, and they give you the stamp "go home" and dont come back. Not all units are heat driven but my guess would be nearly all, as far as overseas likely the same as chems are $$$....
I have built ALOT of stuff from overseas pallets especially tile pallets from third world countries, and some are really cool woods,.....I have NEVER seen an international pallet that was treated, IMHO. I did alot of this while serving in the military when I didnt have alot of extra money..... Some of this "dunnage" is actually pit sawn as we did in this country 100 years ago...
I think the challenge is the lack of BTU density of the pallets, but if you dont have access to cordwood, it'll heat your place, esp. if near or in a city....