You mean the paper-like inner wrap that looks like ham? Think of it this way. It was intended to spend a long time in contact with the ham. And it was known it would be wet. In other words, plenty of opportunity to break down or leach anything bad into the meat. But they aren't going to use anything with that potential. If it can sit there that long against the ham, it's unlikely that heat is going to suddenly produce something harmful.
For that matter, it's well known that people do mess up and leave them on sometimes, and no meat company lawyer is going to let them use something that becomes harmful during heating.
"Kitchen duty is awarded only to those of manifest excellence..." - The Master, Dogen