Those that don’t eat it hold it IS treif(or rather it may be treif and therefore treat it as such), that’s why they don’t eat it. It’s got nothing to do with glatt.
Not necessarily.
I don’t eat it because I prefer to maintain a higher standard for myself. But I would NEVER tell someone who was eating HN that he was eating treif. The same goes for any number of lesser-known hashgachos.
The Wolf